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<div class="section" id="introduction">
<span id="sec-introduction"></span><h1>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h1>
<div class="section" id="overview">
<h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h2>
<p><span class="sc">Psi4</span> provides a wide variety of quantum chemical methods using
state-of-the-art numerical methods and algorithms. Several parts of
the code feature shared-memory parallelization to run efficiently on
multi-core machines (see Sec. <a class="reference internal" href="external.html#sec-threading"><span>Threading</span></a>).
An advanced parser written in Python allows the user
input to have a very simple style for routine computations, but it can also
automate very complex tasks with ease.</p>
<p>In this section, we provide an overview of some of the features of
<span class="sc">Psi4</span> along with the prerequisite steps for running calculations.
Sec. <a class="reference internal" href="tutorial.html#sec-tutorial"><span>Tutorial</span></a> provides a brief tutorial to help new users
get started. Section <a class="reference internal" href="psithoninput.html#sec-psithoninput"><span>Psithon</span></a> offers further details into the
structure of <span class="sc">Psi4</span> input files and how Python can be mixed with
quantum chemistry directives in <span class="sc">Psi4</span>. Section <a class="reference internal" href="psithonfunc.html#sec-psithonfunc"><span>Psithon Functions</span></a>
provides more detail on the Python functions provided by <span class="sc">Psi4</span>
and discusses some of the higher-level functions such as counterpoise
correction, complete-basis-set extrapolation, and running computations
on an entire database of molecules at a time. Later sections deal with
the different types of computations which can be done using <span class="sc">Psi4</span>
(e.g., Hartree–Fock, MP2, coupled-cluster) and general procedures
such as geometry optimization and vibrational frequency analysis.
The <a class="reference internal" href="appendices.html#sec-appendices"><span>Appendices</span></a> include a complete description of all possible input
keywords for each module, as well as tables of available basis sets and
a listing of the sample input files available under <a class="reference external" href="https://github.com/psi4/psi4public/blob/master/samples">psi4/samples</a>.
The user is urged to examine this directory of sample inputs, as
most common types of computations are represented there.
For the latest <span class="sc">Psi4</span> documentation, check
<a class="reference external" href="http://www.psicode.org">www.psicode.org</a>.</p>
</div>
<div class="section" id="citing-psifour">
<h2>Citing <span class="sc">Psi4</span><a class="headerlink" href="#citing-psifour" title="Permalink to this headline">¶</a></h2>
<div class="section" id="overall-psifour-package">
<h3>Overall <span class="sc">Psi4</span> Package<a class="headerlink" href="#overall-psifour-package" title="Permalink to this headline">¶</a></h3>
<p>The following citation should be used in any publication utilizing the
<span class="sc">Psi4</span> program package:</p>
<ul class="simple">
<li>“Psi4: An open-source <em>ab initio</em> electronic structure program,”
J. M. Turney, A. C. Simmonett, R. M. Parrish, E. G. Hohenstein, F.
Evangelista, J. T. Fermann, B. J. Mintz, L. A. Burns, J. J. Wilke, M. L.
Abrams, N. J. Russ, M. L. Leininger, C. L. Janssen, E. T. Seidl, W. D.
Allen, H. F. Schaefer, R. A. King, E. F. Valeev, C. D. Sherrill, and T.
D. Crawford, <em>WIREs Comput. Mol. Sci.</em> <strong>2</strong>, 556 (2012).
(doi: <a class="reference external" href="http://dx.doi.org/10.1002/wcms.93">10.1002/wcms.93</a>).</li>
</ul>
<p>Depending on the particular modules used, the user may also wish to
cite some of the following references for theoretical, algorithmic,
or implementation contributions specific to <span class="sc">Psi4</span> (in addition to
appropriate references for the underlying theory, which are not necessarily
included in the list below).</p>
</div>
<div class="section" id="density-cumulant-functional-theory-dcft">
<h3>Density Cumulant Functional Theory (DCFT)<a class="headerlink" href="#density-cumulant-functional-theory-dcft" title="Permalink to this headline">¶</a></h3>
<p id="intro-dcftcitations"><span class="sc">Psi4</span> features several formulations of newly-developed density cumulant
functional theory (DCFT). The theory and benchmark of this theory are
discussed in the following papers:</p>
<p>DC-06 (also known as DCFT-06):</p>
<ul class="simple">
<li>“Density Cumulant Functional Theory: First Implementation and
Benchmark Results for the DCFT-06 Model,” A. C. Simmonett,
J. J. Wilke, H. F. Schaefer, and W. Kutzelnigg, <em>J. Chem. Phys.</em>
<strong>133</strong>, 174122 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3503657">10.1063/1.3503657</a>).</li>
<li>“Analytic gradients for density cumulant functional theory: The
DCFT-06 model,” A. Yu. Sokolov, J. J. Wilke, A. C. Simmonett,
and H. F. Schaefer, <em>J. Chem. Phys.</em> <strong>137</strong>, 054105 (2012).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4739423">10.1063/1.4739423</a>).</li>
</ul>
<p>DC-12:</p>
<ul class="simple">
<li>“Density cumulant functional theory: The DC-12 method, an improved
description of the one-particle density matrix,” A. Yu. Sokolov,
A. C. Simmonett, and H. F. Schaefer, <em>J. Chem. Phys.</em> <strong>138</strong>, 024107
(2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4773580">10.1063/1.4773580</a>).</li>
</ul>
<p>ODC-06 and ODC-12:</p>
<ul class="simple">
<li>“Orbital-optimized density cumulant functional theory,” A. Yu. Sokolov, and
H. F. Schaefer, <em>J. Chem. Phys.</em> <strong>139</strong>, 204110 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4833138">10.1063/1.4833138</a>).</li>
</ul>
<p>ODC-13:</p>
<ul class="simple">
<li>“Density cumulant functional theory from a unitary transformation:
N-representability, three-particle correlation effects, and application
to O4+,” A. Yu. Sokolov, H. F. Schaefer, and W. Kutzelnigg,
<em>J. Chem. Phys.</em> <strong>141</strong>, 074111 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4892946">10.1063/1.4892946</a>).</li>
</ul>
</div>
<div class="section" id="configuration-interaction-ci">
<h3>Configuration Interaction (CI)<a class="headerlink" href="#configuration-interaction-ci" title="Permalink to this headline">¶</a></h3>
<p>PSI has a highly optimized code for full configuration interaction
and highly correlated configuration interaction, as described in</p>
<ul class="simple">
<li>“The Configuration Interaction Method: Advances in Highly
Correlated Approaches,” C. D. Sherrill and H. F. Schaefer, in
<em>Adv. Quantum Chem.</em>, vol. 34, P.-O. Löwdin, Ed.
(Academic Press, New York, 1999), pp. 143-269.
(doi: <a class="reference external" href="http://dx.doi.org/10.1016/S0065-3276(08)60532-8">10.1016/S0065-3276(08)60532-8</a>).</li>
</ul>
</div>
<div class="section" id="coupled-cluster-cc">
<h3>Coupled Cluster (CC)<a class="headerlink" href="#coupled-cluster-cc" title="Permalink to this headline">¶</a></h3>
<p>A general discussion of coupled cluster theory is given in</p>
<ul class="simple">
<li>“An Introduction to Coupled Cluster Theory for Computational
Chemists,” T. D. Crawford and H. F. Schaefer, <em>Rev. Comp. Chem.</em>
<strong>14</strong>, 33-136 (2000).
(doi: <a class="reference external" href="http://dx.doi.org/10.1002/9780470125915.ch2">10.1002/9780470125915.ch2</a>).</li>
</ul>
<p>Implementation of frozen natural orbital (FNO) coupled cluster theory
in PSI and its performance for non-covalent interactions is discussed
in</p>
<ul class="simple">
<li>“Accurate Noncovalent Interaction Energies Using Truncated Basis Sets
Based on Frozen Natural Orbitals,” A. E. DePrince and C. D. Sherrill,
<em>J. Chem. Theory Comput.</em> <strong>9</strong>, 293-299 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct300780u">10.1021/ct300780u</a>).</li>
</ul>
<p>Implementation of density-fitted (DF) and Cholesky decomposition (CD)
coupled cluster in PSI, and its performance for non-covalent interactions
and reaction energies, is discussed in</p>
<ul class="simple">
<li>“Accuracy and Efficiency of Coupled-Cluster Theory Using
Density Fitting / Cholesky Decomposition, Frozen Natural Orbitals,
and a T1-Transformed Hamiltonian,” A. E. DePrince and C. D. Sherrill,
<em>J. Chem. Theory Comput.</em> <strong>9</strong>, 2687-2696 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct400250u">10.1021/ct400250u</a>).</li>
</ul>
</div>
<div class="section" id="mukherjee-state-specific-multi-reference-coupled-cluster-mk-mrcc">
<h3>Mukherjee State-Specific Multi-Reference Coupled Cluster (Mk-MRCC)<a class="headerlink" href="#mukherjee-state-specific-multi-reference-coupled-cluster-mk-mrcc" title="Permalink to this headline">¶</a></h3>
<p><span class="sc">Psi4</span> features production-level Mukherjee-style state-specific
coupled-cluster theory, including perturbative triples and also associated
multi-reference perturbation theories. The theory and <span class="sc">Psi4</span>
implementation of these methods is discussed in the following papers.</p>
<p>General Mk-MRCC</p>
<ul class="simple">
<li>“Coupling Term Derivation and General Implementation of
State-Specific Multireference Coupled-Cluster Theories,”
F. A. Evangelista, W. D. Allen, and H. F. Schaefer,
<em>J. Chem. Phys.</em> <strong>127</strong>, 024102 (2007).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.2743014">10.1063/1.2743014</a>).</li>
</ul>
<p>Mk-MRCCSD(T)</p>
<ul class="simple">
<li>“Perturbative Triples Corrections in State-Specific Multireference
Coupled Cluster Theory,”
F. A. Evangelista, E. Prochnow, J. Gauss, and H. F. Schaefer,
<em>J. Chem. Phys.</em> <strong>132</strong>, 074107 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3305335">10.1063/1.3305335</a>).</li>
</ul>
<p>Mk-MRCCSDT(-n)</p>
<ul class="simple">
<li>“Triple Excitations in State-Specific Multireference Coupled
Cluster Theory: Application of Mk-MRCCSDT and Mk-MRCCSDT-n Methods to
Model Systems,” F. A. Evangelista, A. C. Simmonett, W. D. Allen,
H. F. Schaefer, and J. Gauss, <em>J. Chem. Phys.</em> <strong>128</strong>, 124104
(2008).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.2834927">10.1063/1.2834927</a>).</li>
</ul>
<p>Mk-MRPT2</p>
<ul class="simple">
<li>“A Companion Perturbation Theory for State-specific
Multireference Coupled Cluster Methods,”
F. A. Evangelista, A. C. Simmonett, H. F. Schaefer, D. Mukherjee, and
W. D. Allen,
<em>Phys. Chem. Chem. Phys.</em> <strong>11</strong>, 4728-4741 (2009).
(doi: <a class="reference external" href="http://dx.doi.org/10.1039/b822910d">10.1039/b822910d</a>).</li>
</ul>
</div>
<div class="section" id="symmetry-adapted-perturbation-theory-sapt">
<h3>Symmetry-Adapted Perturbation Theory (SAPT)<a class="headerlink" href="#symmetry-adapted-perturbation-theory-sapt" title="Permalink to this headline">¶</a></h3>
<p><span class="sc">Psi4</span> features an extremely efficient code to perform wavefunction-based
Symmetry Adapted Perturbation Theory (SAPT). A good review article for this
method is as follows:</p>
<ul class="simple">
<li>“Perturbation Theory Approach to Intermolecular Potential Energy
Surfaces of van der Waals Complexes,” B. Jeziorski, R. Moszynski,
and K. Szalewicz, <em>Chem. Rev.</em> <strong>94</strong>, 1887-1930 (1994).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/cr00031a008">10.1021/cr00031a008</a>).</li>
</ul>
<p><span class="sc">Psi4</span> benefits enormously from the introduction of density fitting (DF)
into SAPT. There are several SAPT truncations available in PSI. For
guidance on which one to choose, see the SAPT section of the manual
and refer to the following systematic study:</p>
<ul class="simple">
<li>“Levels of Symmetry Adapted Perturbation Theory (SAPT). I. Efficiency and
Performance for Interaction Energies,’’ T. M. Parker, L. A. Burns, R. M.
Parrish, A. G. Ryno, and C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>140</strong>,
094106 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4867135">10.1063/1.4867135</a>).</li>
</ul>
<p>The theory and implementation of DF-SAPT is discussed
in the following papers for various levels of SAPT.</p>
<p>DF-SAPT0</p>
<ul class="simple">
<li>“Large-scale Symmetry-adapted Perturbation Theory Computations via
Density Fitting and Laplace Transformation Techniques: Investigating the
Fundamental Forces of DNA-Intercalator Interactions,” E. G. Hohenstein,
R. M. Parrish, C. D. Sherrill, J. M. Turney, and H. F. Schaefer, <em>J.
Chem. Phys.</em> <strong>135</strong>, 174017 (2011).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3656681">10.1063/1.3656681</a>).</li>
<li>“Density Fitting and Cholesky Decomposition Approximations
in Symmetry-Adapted Perturbation Theory: Implementation and Application
to Probe the Nature of <img class="math" src="_images/math/36f9e6d7b6de233f53a202ab02eb63a17958a878.png" alt="\pi - \pi" style="vertical-align: 0px"/> Interactions in Linear Acenes,”
E. G. Hohenstein and C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>132</strong>,
184111 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3426316">10.1063/1.3426316</a>).</li>
</ul>
<p>SAPT2</p>
<ul class="simple">
<li>“Density Fitting of Intramonomer Correlation Effects in
Symmetry-Adapted Perturbation Theory,”
E. G. Hohenstein and C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>133</strong>,
014101 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3451077">10.1063/1.3451077</a>).</li>
</ul>
<p>SAPT2+, SAPT2+(3), SAPT2+3</p>
<ul class="simple">
<li>“Wavefunction Methods for Noncovalent Interactions,” E. G.
Hohenstein and C. D. Sherrill, <em>WIREs: Comput. Mol. Sci.</em> <strong>2</strong>,
304-326 (2012).
(doi: <a class="reference external" href="http://dx.doi.org/10.1002/wcms.84">10.1002/wcms.84</a>).</li>
<li>“Density Fitting of Intramonomer Correlation Effects in
Symmetry-Adapted Perturbation Theory,”
E. G. Hohenstein and C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>133</strong>,
014101 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3451077">10.1063/1.3451077</a>).</li>
<li>“Efficient Evaluation of Triple Excitations in Symmetry-Adapted
Perturbation Theory via MP2 Natural Orbitals,” E. G. Hohenstein
and C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>133</strong>, 104107 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3479400">10.1063/1.3479400</a>).</li>
</ul>
<p>SAPT2+(CCD), SAPT2+(3)(CCD), and SAPT2+3(CCD)</p>
<ul class="simple">
<li>“Tractability Gains in Symmetry-Adapted Perturbation Theory Including
Coupled Double Excitations: CCD+ST(CCD) Dispersion with Natural Orbital
Truncations,’’ R. M. Parrish, E. G. Hohenstein, and C. D. Sherrill,
<em>J. Chem. Phys.</em> <strong>139</strong>, 174102 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4826520">10.1063/1.4826520</a>).</li>
<li>“Wavefunction Methods for Noncovalent Interactions,” E. G.
Hohenstein and C. D. Sherrill, <em>WIREs: Comput. Mol. Sci.</em> <strong>2</strong>,
304-326 (2012).
(doi: <a class="reference external" href="http://dx.doi.org/10.1002/wcms.84">10.1002/wcms.84</a>).</li>
<li>“Density Fitting of Intramonomer Correlation Effects in
Symmetry-Adapted Perturbation Theory,”
E. G. Hohenstein and C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>133</strong>,
014101 (2010).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3451077">10.1063/1.3451077</a>).</li>
</ul>
</div>
<div class="section" id="orbital-optimized-post-hartree-fock-methods">
<h3>Orbital-Optimized Post-Hartree–Fock Methods<a class="headerlink" href="#orbital-optimized-post-hartree-fock-methods" title="Permalink to this headline">¶</a></h3>
<p>Orbital-optimized second-order perturbation theory (OMP2)</p>
<ul class="simple">
<li>“Quadratically convergent algorithm for orbital optimization in the
orbital-optimized coupled-cluster doubles method and in orbital-optimized
second-order Møller–Plesset perturbation theory,”
U. Bozkaya, J. M. Turney, Y. Yamaguchi, H. F. Schaefer, and C. D. Sherrill,
<em>J. Chem. Phys.</em> <strong>135</strong>, 104103 (2011).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3631129">10.1063/1.3631129</a>).</li>
<li>“Analytic energy gradients for the orbital-optimized second-order
Møller–Plesset perturbation theory,” U. Bozkaya and
C. D. Sherrill, <em>J. Chem. Phys.</em> <strong>138</strong>, 184103 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4803662">10.1063/1.4803662</a>).</li>
<li>“Orbital-Optimized Second-Order Perturbation Theory with Density-Fitting
and Cholesky Decomposition Approximations: An Efficient Implementation,”
U. Bozkaya, <em>J. Chem. Theory Comput.</em> <strong>10</strong>, 2371 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct500231c">10.1021/ct500231c</a>).</li>
</ul>
<p>Orbital-optimized third-order perturbation theory (OMP3)</p>
<ul class="simple">
<li>“Orbital-Optimized Third-Order Møller–Plesset Perturbation
Theory and Its Spin-Component and Spin-Opposite Scaled Variants: Application
to Symmetry Breaking Problems,” U. Bozkaya,
<em>J. Chem. Phys.</em> <strong>135</strong>, 224103 (2011).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.3665134">10.1063/1.3665134</a>).</li>
<li>“Assessment of Orbital-Optimized Third-Order Møller–Plesset
Perturbation Theory and Its Spin-Component and Spin-Opposite Scaled Variants
for Thermochemistry and Kinetics,” E. Soydas and U. Bozkaya,
<em>J. Chem. Theory Comput.</em> <strong>9</strong>, 1452 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct301078q">10.1021/ct301078q</a>).</li>
<li>“Analytic energy gradients for the orbital-optimized third-order Møller–Plesset
Perturbation Theory,” U. Bozkaya,
<em>J. Chem. Phys.</em> <strong>139</strong>, 104116 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4820877">10.1063/1.4820877</a>).</li>
</ul>
<p>Orbital-optimized coupled electron pair approximation (OCEPA)</p>
<ul class="simple">
<li>“Orbital-optimized coupled-electron pair theory and its analytic gradients:
Accurate equilibrium geometries, harmonic vibrational frequencies, and hydrogen transfer
reactions,” U. Bozkaya and C. D. Sherrill,
<em>J. Chem. Phys.</em> <strong>139</strong>, 054104 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4816628">10.1063/1.4816628</a>).</li>
</ul>
<p>Orbital-optimized MP2.5 (OMP2.5)</p>
<ul class="simple">
<li>“Orbital-optimized MP2.5 and its analytic gradients: Approaching CCSD(T)
quality for noncovalent interactions,” U. Bozkaya and C. D. Sherrill,
<em>J. Chem. Phys.</em> <strong>141</strong>, 204105 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4902226">10.1063/1.4902226</a>).</li>
</ul>
<p>Extended Koopmans’ Theorem</p>
<ul class="simple">
<li>“The extended Koopmans’ theorem for orbital-optimized methods: Accurate
computation of ionization potentials,” U. Bozkaya, <em>J. Chem. Phys.</em>
<strong>139</strong>, 154105 (2013).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.4825041">10.1063/1.4825041</a>).</li>
<li>“Accurate Electron Affinities from the Extended Koopmans’ Theorem Based on Orbital-Optimized Methods,”
U. Bozkaya, <em>J. Chem. Theory Comput.</em> <strong>10</strong>, 2041 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct500186j">10.1021/ct500186j</a>).</li>
</ul>
<p>Density-Fitted Orbital-optimized second-order perturbation theory (DF-OMP2)</p>
<ul class="simple">
<li>“Orbital-Optimized Second-Order Perturbation Theory with Density-Fitting
and Cholesky Decomposition Approximations: An Efficient Implementation,”
U. Bozkaya, <em>J. Chem. Theory Comput.</em> <strong>10</strong>, 2371 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct500231c">10.1021/ct500231c</a>).</li>
<li>“Analytic Energy Gradients and Spin Multiplicities for Orbital-Optimized
Second-Order Perturbation Theory with Density-Fitting Approximation: An
Efficient Implementation,” U. Bozkaya, <em>J. Chem. Theory Comput.</em> <strong>10</strong>, 4389 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1021/ct500634s">10.1021/ct500634s</a>).</li>
</ul>
</div>
<div class="section" id="second-order-algebraic-diagrammatic-construction-adc-2">
<h3>Second-Order Algebraic-Diagrammatic Construction [ADC(2)]<a class="headerlink" href="#second-order-algebraic-diagrammatic-construction-adc-2" title="Permalink to this headline">¶</a></h3>
<p>General ADC(2) theory</p>
<ul class="simple">
<li>“Intermediate state representation approach to physical properties of
electronically excited molecules,”
J. Schirmer, and A. B. Trofimov, <em>J. Chem. Phys.</em> <strong>120</strong>,
11449-11464 (2004).
(doi: <a class="reference external" href="http://dx.doi.org/10.1063/1.1752875">10.1063/1.1752875</a>).</li>
</ul>
<p>Theory of “Partially-renormalized” CIS(D) and ADC(2) [PR-CIS(D) and PR-ADC(2)]
and their implementation in <span class="sc">Psi4</span></p>
<ul class="simple">
<li>“Excited State Calculation for Free-Base and Metalloporphyrins with
the Partially Renormalized Polarization Propagator Approach,”
M. Saitow and Y. Mochizuki, <em>Chem. Phys. Lett.</em> <strong>525</strong>, 144-149
(2012).
(doi: <a class="reference external" href="http://dx.doi.org/10.1016/j.cplett.2011.12.063">10.1016/j.cplett.2011.12.063</a>).</li>
</ul>
</div>
<div class="section" id="density-matrix-renormalization-group-dmrg">
<h3>Density Matrix Renormalization Group (DMRG)<a class="headerlink" href="#density-matrix-renormalization-group-dmrg" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li>“CheMPS2: a free open-source spin-adapted implementation of the density
matrix renormalization group for ab initio quantum chemistry,”
S. Wouters, W. Poelmans, P. W. Ayers and D. Van Neck,
<em>Comput. Phys. Commun.</em> <strong>185</strong> (6), 1501-1514 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1016/j.cpc.2014.01.019">10.1016/j.cpc.2014.01.019</a>).</li>
<li>“The density matrix renormalization group for ab initio quantum chemistry,”
S. Wouters and D. Van Neck, <em>Eur. Phys. J. D</em> <strong>68</strong> (9), 272 (2014).
(doi: <a class="reference external" href="http://dx.doi.org/10.1140/epjd/e2014-50500-1">10.1140/epjd/e2014-50500-1</a>).</li>
</ul>
<span class="target" id="index-0"></span></div>
</div>
<div class="section" id="supported-architectures">
<span id="index-1"></span><h2>Supported Architectures<a class="headerlink" href="#supported-architectures" title="Permalink to this headline">¶</a></h2>
<p>The majority of <span class="sc">Psi4</span> was developed on Mac and Linux machines. In
principle, it should work on any Unix system; however, we have not tested
extensively on systems other than Mac and Linux. There is not a Windows
version of <span class="sc">Psi4</span>.</p>
<p><span class="sc">Psi4</span> has been successfully compiled using Intel, GCC, and Clang
compilers. For the Intel compilers, we recommend at least 12.1 (we have
had trouble with version 12.0 and 13.0.1). GCC version 4.6 or above is
recommended. For some architectures, a <a class="reference internal" href="conda.html#sec-conda"><span>precompiled binary</span></a> is available. See <a class="reference internal" href="external.html#sec-installfile"><span>Compiling and Installing</span></a> for details.</p>
</div>
<div class="section" id="capabilities">
<h2>Capabilities<a class="headerlink" href="#capabilities" title="Permalink to this headline">¶</a></h2>
<p><span class="sc">Psi4</span> can perform <em>ab initio</em> computations employing basis
sets of contrated Gaussian-type functions of virtually arbitrary
orbital quantum number. Many parts of <span class="sc">Psi4</span> can recognize and
exploit the largest Abelian subgroup of the molecular point group.
Table <a class="reference internal" href="#table-methods"><span>Methods</span></a> displays the range of theoretical methods
available in <span class="sc">Psi4</span>.
For more details, see Tables <a class="reference internal" href="energy.html#table-energy-gen"><span>Energy</span></a>,
<a class="reference internal" href="energy.html#table-energy-dft"><span>Energy (DFT)</span></a>, <a class="reference internal" href="mrcc_table_energy.html#table-energy-mrcc"><span>Energy (MRCC)</span></a>,
<a class="reference internal" href="energy.html#table-energy-cfour"><span>Energy (CFOUR)</span></a>, <a class="reference internal" href="opt.html#table-grad-gen"><span>Gradient</span></a>,
<a class="reference internal" href="opt.html#table-grad-cfour"><span>Gradient (CFOUR)</span></a>, and <a class="reference internal" href="freq.html#table-freq-gen"><span>Frequency</span></a>.</p>
<table border="1" class="docutils" id="id54">
<span id="table-methods"></span><caption><span class="caption-text">Summary of theoretical methods available in <span class="sc">Psi4</span></span><a class="headerlink" href="#id54" title="Permalink to this table">¶</a></caption>
<colgroup>
<col width="26%" />
<col width="11%" />
<col width="11%" />
<col width="22%" />
<col width="30%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Method</th>
<th class="head">Energy</th>
<th class="head">Gradient</th>
<th class="head">Reference</th>
<th class="head">Parallelism</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>SCF (HF and DFT)</td>
<td>Y</td>
<td>Y <a class="footnote-reference" href="#f4" id="id27">[4]</a></td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>threaded</td>
</tr>
<tr class="row-odd"><td>DF-SCF (HF and DFT)</td>
<td>Y</td>
<td>Y <a class="footnote-reference" href="#f4" id="id28">[4]</a></td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>threaded</td>
</tr>
<tr class="row-even"><td>EFP <a class="footnote-reference" href="#f5" id="id29">[5]</a></td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td> </td>
</tr>
<tr class="row-odd"><td>DCFT</td>
<td>Y</td>
<td>Y</td>
<td>UHF</td>
<td>partially threaded</td>
</tr>
<tr class="row-even"><td>MP2</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id30">[3]</a></td>
</tr>
<tr class="row-odd"><td>DF-MP2</td>
<td>Y</td>
<td>Y <a class="footnote-reference" href="#f2" id="id31">[2]</a></td>
<td>RHF/ROHF/UHF</td>
<td>threaded</td>
</tr>
<tr class="row-even"><td>DF-MP3</td>
<td>Y</td>
<td>—</td>
<td>RHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id32">[3]</a></td>
</tr>
<tr class="row-odd"><td>MP3</td>
<td>Y</td>
<td>Y</td>
<td>RHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id33">[3]</a></td>
</tr>
<tr class="row-even"><td>MP2.5</td>
<td>Y</td>
<td>Y</td>
<td>RHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id34">[3]</a></td>
</tr>
<tr class="row-odd"><td>MP4</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id35">[3]</a></td>
</tr>
<tr class="row-even"><td>MP(n)</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF</td>
<td>partially threaded</td>
</tr>
<tr class="row-odd"><td>ZAPT(n)</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF</td>
<td>partially threaded</td>
</tr>
<tr class="row-even"><td>OMP2</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>partially threaded</td>
</tr>
<tr class="row-odd"><td>OMP3</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>partially threaded</td>
</tr>
<tr class="row-even"><td>OMP2.5</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>partially threaded</td>
</tr>
<tr class="row-odd"><td>DF-OMP2</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id36">[3]</a></td>
</tr>
<tr class="row-even"><td>OCEPA</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF/RKS/UKS</td>
<td>partially threaded</td>
</tr>
<tr class="row-odd"><td>CEPA(0)</td>
<td>Y</td>
<td>Y</td>
<td>RHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id37">[3]</a></td>
</tr>
<tr class="row-even"><td>CEPA(n), n=0,1,3</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id38">[3]</a></td>
</tr>
<tr class="row-odd"><td>ACPF/AQCC</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id39">[3]</a></td>
</tr>
<tr class="row-even"><td>QCISD</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id40">[3]</a></td>
</tr>
<tr class="row-odd"><td>QCISD(T)</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id41">[3]</a></td>
</tr>
<tr class="row-even"><td>CC2</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id42">[3]</a></td>
</tr>
<tr class="row-odd"><td>CCSD</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id43">[3]</a></td>
</tr>
<tr class="row-even"><td>DF-CCSD</td>
<td>Y</td>
<td>Y</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id44">[3]</a></td>
</tr>
<tr class="row-odd"><td>DF-CCD</td>
<td>Y</td>
<td>Y</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id45">[3]</a></td>
</tr>
<tr class="row-even"><td>CCSD(T)</td>
<td>Y</td>
<td>Y <a class="footnote-reference" href="#f1" id="id46">[1]</a></td>
<td>RHF/ROHF/UHF</td>
<td>threaded (pthreads)</td>
</tr>
<tr class="row-odd"><td>DF-CCSD(T)</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id47">[3]</a></td>
</tr>
<tr class="row-even"><td>CC3</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF/UHF</td>
<td>threaded (pthreads)</td>
</tr>
<tr class="row-odd"><td>Mk-MRPT2</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF/TCSCF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id48">[3]</a></td>
</tr>
<tr class="row-even"><td>Mk-MRCCSD</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF/TCSCF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id49">[3]</a></td>
</tr>
<tr class="row-odd"><td>Mk-MRCCSD(T)</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF/TCSCF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id50">[3]</a></td>
</tr>
<tr class="row-even"><td>CI(n)</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF</td>
<td>partially threaded</td>
</tr>
<tr class="row-odd"><td>RAS-CI</td>
<td>Y</td>
<td>—</td>
<td>RHF/ROHF</td>
<td>partially threaded</td>
</tr>
<tr class="row-even"><td>SAPT</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded</td>
</tr>
<tr class="row-odd"><td>CIS/RPA/TDHF</td>
<td>Y</td>
<td>—</td>
<td> </td>
<td> </td>
</tr>
<tr class="row-even"><td>ADC(2)</td>
<td>Y</td>
<td>—</td>
<td>RHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id51">[3]</a></td>
</tr>
<tr class="row-odd"><td>EOM-CCSD</td>
<td>Y</td>
<td>Y</td>
<td>RHF/ROHF/UHF</td>
<td>threaded <a class="footnote-reference" href="#f3" id="id52">[3]</a></td>
</tr>
<tr class="row-even"><td>DMRG-CI</td>
<td>Y</td>
<td>—</td>
<td> </td>
<td> </td>
</tr>
<tr class="row-odd"><td>DMRG-SCF</td>
<td>Y</td>
<td>—</td>
<td> </td>
<td> </td>
</tr>
</tbody>
</table>
<p>Geometry optimization can be performed using either analytic gradients
or energy points. Likewise, vibrational frequencies can be
computed by analytic second derivatives, by finite
differences of analytic gradients, or by finite differences of energies.
<span class="sc">Psi4</span> can also compute an extensive list of one-electron properties.</p>
</div>
<div class="section" id="technical-support">
<span id="index-2"></span><h2>Technical Support<a class="headerlink" href="#technical-support" title="Permalink to this headline">¶</a></h2>
<p>The <span class="sc">Psi4</span> package is
distributed for free and without any guarantee of reliability,
accuracy, or suitability for any particular purpose. No obligation
to provide technical support is expressed or implied. As time
allows, the developers will attempt to answer inquiries directed to
<a class="reference external" href="mailto:crawdad%40vt.edu">crawdad<span>@</span>vt<span>.</span>edu</a>
or <a class="reference external" href="mailto:sherrill%40gatech.edu">sherrill<span>@</span>gatech<span>.</span>edu</a>.
For bug reports, specific and detailed information, with example
inputs, would be appreciated. Questions or comments regarding
this user’s manual may be sent to
<a class="reference external" href="mailto:sherrill%40gatech.edu">sherrill<span>@</span>gatech<span>.</span>edu</a>.</p>
<p>Alternatively, bug reports and comments can be submitted to the <a class="reference external" href="https://github.com/psi4/psi4public/issues/new">Issue
tracker on GitHub</a> . This site
is viewable by all, but reporting bugs requires signing up for a <a class="reference external" href="https://github.com/signup/free">free
GitHub account</a>.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="f1" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id46">[1]</a></td><td>UHF-CCSD(T) gradients only, as of devel</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="f2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id31">[2]</a></td><td>Gradients are available for RHF and UHF references.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="f3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[3]</td><td><em>(<a class="fn-backref" href="#id30">1</a>, <a class="fn-backref" href="#id32">2</a>, <a class="fn-backref" href="#id33">3</a>, <a class="fn-backref" href="#id34">4</a>, <a class="fn-backref" href="#id35">5</a>, <a class="fn-backref" href="#id36">6</a>, <a class="fn-backref" href="#id37">7</a>, <a class="fn-backref" href="#id38">8</a>, <a class="fn-backref" href="#id39">9</a>, <a class="fn-backref" href="#id40">10</a>, <a class="fn-backref" href="#id41">11</a>, <a class="fn-backref" href="#id42">12</a>, <a class="fn-backref" href="#id43">13</a>, <a class="fn-backref" href="#id44">14</a>, <a class="fn-backref" href="#id45">15</a>, <a class="fn-backref" href="#id47">16</a>, <a class="fn-backref" href="#id48">17</a>, <a class="fn-backref" href="#id49">18</a>, <a class="fn-backref" href="#id50">19</a>, <a class="fn-backref" href="#id51">20</a>, <a class="fn-backref" href="#id52">21</a>)</em> threading through BLAS routines only</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="f4" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[4]</td><td><em>(<a class="fn-backref" href="#id27">1</a>, <a class="fn-backref" href="#id28">2</a>)</em> DFT gradients only implemented for SCF type DF. LRC-DFT gradients not implemented yet.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="f5" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id29">[5]</a></td><td>Both EFP/EFP and QM/EFP energies are available.</td></tr>
</tbody>
</table>
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<h3><a href="index.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">Introduction</a><ul>
<li><a class="reference internal" href="#overview">Overview</a></li>
<li><a class="reference internal" href="#citing-psifour">Citing <span class="sc">Psi4</span></a><ul>
<li><a class="reference internal" href="#overall-psifour-package">Overall <span class="sc">Psi4</span> Package</a></li>
<li><a class="reference internal" href="#density-cumulant-functional-theory-dcft">Density Cumulant Functional Theory (DCFT)</a></li>
<li><a class="reference internal" href="#configuration-interaction-ci">Configuration Interaction (CI)</a></li>
<li><a class="reference internal" href="#coupled-cluster-cc">Coupled Cluster (CC)</a></li>
<li><a class="reference internal" href="#mukherjee-state-specific-multi-reference-coupled-cluster-mk-mrcc">Mukherjee State-Specific Multi-Reference Coupled Cluster (Mk-MRCC)</a></li>
<li><a class="reference internal" href="#symmetry-adapted-perturbation-theory-sapt">Symmetry-Adapted Perturbation Theory (SAPT)</a></li>
<li><a class="reference internal" href="#orbital-optimized-post-hartree-fock-methods">Orbital-Optimized Post-Hartree–Fock Methods</a></li>
<li><a class="reference internal" href="#second-order-algebraic-diagrammatic-construction-adc-2">Second-Order Algebraic-Diagrammatic Construction [ADC(2)]</a></li>
<li><a class="reference internal" href="#density-matrix-renormalization-group-dmrg">Density Matrix Renormalization Group (DMRG)</a></li>
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<li><a class="reference internal" href="#supported-architectures">Supported Architectures</a></li>
<li><a class="reference internal" href="#capabilities">Capabilities</a></li>
<li><a class="reference internal" href="#technical-support">Technical Support</a></li>
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