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<span class="target" id="index-0"></span><div class="section" id="ci-configuration-interaction">
<span id="sec-ci"></span><span id="index-1"></span><h1>CI: Configuration Interaction<a class="headerlink" href="#ci-configuration-interaction" title="Permalink to this headline">¶</a></h1>
<p><em>Code author: C. David Sherrill and Matthew L. Leininger</em></p>
<p><em>Section author: C. David Sherrill</em></p>
<p><em>Module:</em> <a class="reference internal" href="autodir_options_c/module__detci.html#apdx-detci"><span>Keywords</span></a>, <a class="reference internal" href="autodir_psivariables/module__detci.html#apdx-detci-psivar"><span>PSI Variables</span></a>, <a class="reference external" href="https://github.com/psi4/psi4public/blob/master/src/bin/detci">DETCI</a></p>
<p>Configuration interaction (CI) is one of the most general ways to
improve upon Hartree–Fock theory by adding a description of the
correlated motions of electrons. Simply put, a CI wavefunction
is a linear combination of Slater determinants (or spin-adapted
configuration state functions), with the linear coefficients being
determined variationally via diagonalization of the Hamiltonian in the
given subspace of determinants. For a “single-reference” CI based
on reference function <img class="math" src="_images/math/b4d1fc88a2082a1784f8ccb00fefd243279f3b08.png" alt="| \Phi_0 \rangle" style="vertical-align: -5px"/>, we can write the CI expansion as
follows:</p>
<div class="math" id="equation-CIexpansion">
<p><span class="eqno">(1)</span><img src="_images/math/4852f7cc0c951de3d75f8061949ae07fb3997f24.png" alt="| \Psi \rangle = c_0 | \Phi_0 \rangle
+ \sum_i^{\rm occ} \sum_a^{\rm vir} c_i^a | \Phi_i^a \rangle
+ \sum_{i<j}^{\rm occ} \sum_{a<b}^{\rm vir} c_{ij}^{ab}
| \Phi_{ij}^{ab} \rangle
+ \sum_{i<j<k}^{\rm occ} \sum_{a<b<c}^{\rm vir} c_{ijk}^{abc}
| \Phi_{ijk}^{abc} \rangle + \cdots"/></p>
</div><p>The simplest standard CI method that improves upon Hartree–Fock is a CI
that adds all singly <img class="math" src="_images/math/a3de3ea20cd320eacac1c0b9b9af9d82fe5e93b9.png" alt="| \Phi_i^a \rangle" style="vertical-align: -5px"/> and doubly
<img class="math" src="_images/math/1a0a5ea8fc47b0f196004585d4002c7660f114ce.png" alt="| \Phi_{ij}^{ab} \rangle" style="vertical-align: -8px"/>
substituted determinants (CISD) to the reference determinant
<img class="math" src="_images/math/b4d1fc88a2082a1784f8ccb00fefd243279f3b08.png" alt="| \Phi_0 \rangle" style="vertical-align: -5px"/>. The CISD wavefunction has fallen out of favor
because truncated CI wavefunctions are not size-extensive, meaning
that their quality degrades for larger molecules. MP2 is a less
expensive alternative giving results similar to those of CISD for small
molecules, but the quality of MP2 does not degrade for larger molecules.
Coupled-cluster singles and doubles (CCSD) is another size-extensive
alternative; it is only slightly more costly computationally than CISD,
but it typically provides significantly more accurate results.</p>
<p>The CI code in <span class="sc">Psi4</span> is described in detail in
<a class="reference internal" href="bibliography.html#sherrill-1999-ci" id="id1">[Sherrill:1999:CI]</a>. For the reasons stated above, the CI code in
<span class="sc">Psi4</span> is not optimized for CISD computations. Instead, emphasis
has been placed on developing a very efficient program to handle more
general CI wavefunctions which may be helpful in more challenging cases
such as highly strained molecules or bond breaking reactions. The CI
code is based on the fast, determinant-based string formalism
of Handy <a class="reference internal" href="bibliography.html#handy-1980" id="id2">[Handy:1980]</a>. It can solve for restricted active space
configuration interaction (RAS CI) wavefunctions as described by Olsen,
Roos, Jorgensen, and Aa. Jensen <a class="reference internal" href="bibliography.html#olsen-1988" id="id3">[Olsen:1988]</a>. Excitation-class
selected multi-reference CI wavefunctions, such as second-order CI,
can be formulated as RAS CI’s. A RAS CI selects determinants for the
model space as those which have no more than <img class="math" src="_images/math/3492c6d4b042ddb93d57b7a5aa9532131d9af611.png" alt="n" style="vertical-align: 0px"/> holes in the lowest set
of orbitals (called RAS I) and no more than <img class="math" src="_images/math/47e031122c1df8bef662b93f6a2c0751959a98d1.png" alt="m" style="vertical-align: 0px"/> electrons in the highest
set of orbitals (called RAS III). An intermediate set of orbitals, if
present (RAS II), has no restrictions placed upon it. All determinants
satisfying these rules are included in the CI.</p>
<p>The DETCI module is also very efficient at computing full configuration
interaction
wavefunctions, and it is used in this capacity in the complete-active-space
self-consistent-field (CASSCF) code. Use of DETCI for CASSCF
wavefunctions is described in another section of this manual.</p>
<p>As just mentioned, the DETCI module is designed for challenging
chemical systems for which simple CISD is not suitable. Because
CI wavefunctions which go beyond CISD (such as RAS CI) are fairly complex,
typically the DETCI code will be used in cases where the
tradeoffs between computational expense and completeness of the
model space are nontrivial. Hence, the user is advised to develop
a good working knowledge of multi-reference and RAS CI methods before
attempting to use the program for a production-level project. This user’s
manual will provide only an elementary introduction to the most
important keywords. Additional information is available in the complete
list of keywords for DETCI provided in Appendix <a class="reference internal" href="autodir_options_c/module__detci.html#apdx-detci"><span>DETCI</span></a>.</p>
<p>The division of the molecular orbitals into various subspaces such as
RAS spaces, or frozen <em>vs.</em> active orbitals, <em>etc.</em>, needs to be clear not
only to detci, but also at least to the transformation program
(and in the case of MCSCF, to other programs as well). Thus, orbital
subspace keywords such as <a class="reference internal" href="autodoc_glossary_options_c.html#term-ras1-globals"><span class="xref std std-term">RAS1</span></a>,
<a class="reference internal" href="autodoc_glossary_options_c.html#term-ras2-globals"><span class="xref std std-term">RAS2</span></a>, <a class="reference internal" href="autodoc_glossary_options_c.html#term-ras3-globals"><span class="xref std std-term">RAS3</span></a>, <a class="reference internal" href="autodoc_glossary_options_c.html#term-frozen-docc-globals"><span class="xref std std-term">FROZEN_DOCC</span></a>,
<a class="reference internal" href="autodoc_glossary_options_c.html#term-frozen-uocc-globals"><span class="xref std std-term">FROZEN_UOCC</span></a>,
<a class="reference internal" href="autodoc_glossary_options_c.html#term-active-globals"><span class="xref std std-term">ACTIVE</span></a>, etc., should be set
in the global section of input so they may also be read by other modules.</p>
<p>For single-reference CI computations, the easiest way to invoke a CI
computation with DETCI is simply to call <code class="docutils literal"><span class="pre">energy()</span></code>, <code class="docutils literal"><span class="pre">optimize()</span></code>, etc.,
with the common name for that CI wavefunction, like <code class="docutils literal"><span class="pre">energy('cisd')</span></code>
for a CISD single-point energy. The Python driver
recognizes <code class="docutils literal"><span class="pre">cisd</span></code>, <code class="docutils literal"><span class="pre">cisdt</span></code>, and <code class="docutils literal"><span class="pre">cisdtq</span></code>. Higher order
single-refernce CI wavefunctions, like those including singles through
6-fold excitations, can be invoked using numbers, like <code class="docutils literal"><span class="pre">ci6</span></code>. A full
CI can be specifed by <code class="docutils literal"><span class="pre">fci</span></code>. More complicated CI computations, like
RASCI, can be performed by setting the appropriate keywords and calling the
module generically like <code class="docutils literal"><span class="pre">energy('detci')</span></code>. The latter approach
will also work for any of the previously-mentioned CI wavefunctions for
which the driver has built-in shortcuts, so long as the relevant options
(especially <a class="reference internal" href="autodoc_glossary_options_c.html#term-ex-level-detci"><span class="xref std std-term">EX_LEVEL</span></a>) are set appropriately. Some
examples of single-refence CI, RASCI, and full CI computations are provided
in <a class="reference external" href="https://github.com/psi4/psi4public/blob/master/samples">psi4/samples</a>.</p>
<div class="section" id="basic-detci-keywords">
<span id="index-2"></span><h2>Basic DETCI Keywords<a class="headerlink" href="#basic-detci-keywords" title="Permalink to this headline">¶</a></h2>
<div class="section" id="reference">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-reference-detci"><span class="xref std std-term">REFERENCE</span></a><a class="headerlink" href="#reference" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Reference wavefunction type</p>
<ul class="simple">
<li><strong>Type</strong>: string</li>
<li><strong>Possible Values</strong>: RHF, ROHF</li>
<li><strong>Default</strong>: RHF</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="r-convergence">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-r-convergence-detci"><span class="xref std std-term">R_CONVERGENCE</span></a><a class="headerlink" href="#r-convergence" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Convergence criterion for CI residual vector in the Davidson algorithm (RMS error). The default is 1e-4 for energies and 1e-7 for gradients.</p>
<ul class="simple">
<li><strong>Type</strong>: <a class="reference internal" href="notes_c.html#op-c-conv"><span>conv double</span></a></li>
<li><strong>Default</strong>: 1e-4</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="ex-level">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-ex-level-detci"><span class="xref std std-term">EX_LEVEL</span></a><a class="headerlink" href="#ex-level" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>The CI excitation level</p>
<ul class="simple">
<li><strong>Type</strong>: integer</li>
<li><strong>Default</strong>: 2</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="fci">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-fci-detci"><span class="xref std std-term">FCI</span></a><a class="headerlink" href="#fci" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Do a full CI (FCI)? If TRUE, overrides the value of <a class="reference internal" href="autodoc_glossary_options_c.html#term-ex-level-detci"><span class="xref std std-term">EX_LEVEL</span></a></p>
<ul class="simple">
<li><strong>Type</strong>: <a class="reference internal" href="notes_c.html#op-c-boolean"><span>boolean</span></a></li>
<li><strong>Default</strong>: false</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="frozen-docc">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-frozen-docc-globals"><span class="xref std std-term">FROZEN_DOCC</span></a><a class="headerlink" href="#frozen-docc" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>An array containing the number of frozen doubly-occupied orbitals per irrep (these are not excited in a correlated wavefunction, nor can they be optimized in MCSCF. This trumps <a class="reference internal" href="autodoc_glossary_options_c.html#term-num-frozen-docc-globals"><span class="xref std std-term">NUM_FROZEN_DOCC</span></a> and <a class="reference internal" href="autodoc_glossary_options_c.html#term-freeze-core-globals"><span class="xref std std-term">FREEZE_CORE</span></a></p>
<ul class="simple">
<li><strong>Type</strong>: array</li>
<li><strong>Default</strong>: No Default</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="restricted-docc">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-restricted-docc-globals"><span class="xref std std-term">RESTRICTED_DOCC</span></a><a class="headerlink" href="#restricted-docc" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>An array giving the number of restricted doubly-occupied orbitals per irrep (not excited in CI wavefunctions, but orbitals can be optimized in MCSCF)</p>
<ul class="simple">
<li><strong>Type</strong>: array</li>
<li><strong>Default</strong>: No Default</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="restricted-uocc">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-restricted-uocc-globals"><span class="xref std std-term">RESTRICTED_UOCC</span></a><a class="headerlink" href="#restricted-uocc" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>An array giving the number of restricted unoccupied orbitals per irrep (not occupied in CI wavefunctions, but orbitals can be optimized in MCSCF)</p>
<ul class="simple">
<li><strong>Type</strong>: array</li>
<li><strong>Default</strong>: No Default</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="frozen-uocc">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-frozen-uocc-globals"><span class="xref std std-term">FROZEN_UOCC</span></a><a class="headerlink" href="#frozen-uocc" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>An array containing the number of frozen unoccupied orbitals per irrep (these are not populated in a correlated wavefunction, nor can they be optimized in MCSCF. This trumps <a class="reference internal" href="autodoc_glossary_options_c.html#term-num-frozen-uocc-globals"><span class="xref std std-term">NUM_FROZEN_UOCC</span></a></p>
<ul class="simple">
<li><strong>Type</strong>: array</li>
<li><strong>Default</strong>: No Default</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="maxiter">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-maxiter-detci"><span class="xref std std-term">MAXITER</span></a><a class="headerlink" href="#maxiter" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Maximum number of iterations to diagonalize the Hamiltonian</p>
<ul class="simple">
<li><strong>Type</strong>: integer</li>
<li><strong>Default</strong>: 12</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="num-roots">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-num-roots-detci"><span class="xref std std-term">NUM_ROOTS</span></a><a class="headerlink" href="#num-roots" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>number of CI roots to find</p>
<ul class="simple">
<li><strong>Type</strong>: integer</li>
<li><strong>Default</strong>: 1</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="icore">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-icore-detci"><span class="xref std std-term">ICORE</span></a><a class="headerlink" href="#icore" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Specifies how to handle buffering of CI vectors. A value of 0 makes the program perform I/O one RAS subblock at a time; 1 uses entire CI vectors at a time; and 2 uses one irrep block at a time. Values of 0 or 2 cause some inefficiency in the I/O (requiring multiple reads of the C vector when constructing H in the iterative subspace if <a class="reference internal" href="autodoc_glossary_options_c.html#term-diag-method-detci"><span class="xref std std-term">DIAG_METHOD</span></a> = SEM), but require less core memory.</p>
<ul class="simple">
<li><strong>Type</strong>: integer</li>
<li><strong>Default</strong>: 1</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="diag-method">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-diag-method-detci"><span class="xref std std-term">DIAG_METHOD</span></a><a class="headerlink" href="#diag-method" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>This specifies which method is to be used in diagonalizing the Hamiltonian. The valid options are: <code class="docutils literal"><span class="pre">RSP</span></code>, to form the entire H matrix and diagonalize using libciomr to obtain all eigenvalues (n.b. requires HUGE memory); <code class="docutils literal"><span class="pre">OLSEN</span></code>, to use Olsen’s preconditioned inverse subspace method (1990); <code class="docutils literal"><span class="pre">MITRUSHENKOV</span></code>, to use a 2x2 Olsen/Davidson method; and <code class="docutils literal"><span class="pre">DAVIDSON</span></code> (or <code class="docutils literal"><span class="pre">SEM</span></code>) to use Liu’s Simultaneous Expansion Method, which is identical to the Davidson method if only one root is to be found. There also exists a SEM debugging mode, <code class="docutils literal"><span class="pre">SEMTEST</span></code>. The <code class="docutils literal"><span class="pre">SEM</span></code> method is the most robust, but it also requires <img class="math" src="_images/math/3519e1a4d4fadea57bacef6ceb3ba76db785e327.png" alt="2NM+1" style="vertical-align: -1px"/> CI vectors on disk, where <img class="math" src="_images/math/8c45b38d633fb6de83fc7087c4db116a5565752a.png" alt="N" style="vertical-align: 0px"/> is the maximum number of iterations and <img class="math" src="_images/math/b6e6bf2d998d1a33904141106a453324f9c97b35.png" alt="M" style="vertical-align: 0px"/> is the number of roots.</p>
<ul class="simple">
<li><strong>Type</strong>: string</li>
<li><strong>Possible Values</strong>: RSP, OLSEN, MITRUSHENKOV, DAVIDSON, SEM, SEMTEST</li>
<li><strong>Default</strong>: SEM</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="opdm">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-opdm-detci"><span class="xref std std-term">OPDM</span></a><a class="headerlink" href="#opdm" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Do compute one-particle density matrix if not otherwise required?</p>
<ul class="simple">
<li><strong>Type</strong>: <a class="reference internal" href="notes_c.html#op-c-boolean"><span>boolean</span></a></li>
<li><strong>Default</strong>: false</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="tdm">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-tdm-detci"><span class="xref std std-term">TDM</span></a><a class="headerlink" href="#tdm" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Do compute the transition density? Note: only transition densities between roots of the same symmetry will be evaluated. DETCI does not compute states of different irreps within the same computation; to do this, lower the symmetry of the computation.</p>
<ul class="simple">
<li><strong>Type</strong>: <a class="reference internal" href="notes_c.html#op-c-boolean"><span>boolean</span></a></li>
<li><strong>Default</strong>: false</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="dipmom">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-dipmom-detci"><span class="xref std std-term">DIPMOM</span></a><a class="headerlink" href="#dipmom" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Do compute the dipole moment?</p>
<ul class="simple">
<li><strong>Type</strong>: <a class="reference internal" href="notes_c.html#op-c-boolean"><span>boolean</span></a></li>
<li><strong>Default</strong>: false</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="mpn">
<h3><a class="reference internal" href="autodoc_glossary_options_c.html#term-mpn-detci"><span class="xref std std-term">MPN</span></a><a class="headerlink" href="#mpn" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p>Do compute the MPn series out to kth order where k is determined by <a class="reference internal" href="autodoc_glossary_options_c.html#term-max-num-vecs-detci"><span class="xref std std-term">MAX_NUM_VECS</span></a> ? For open-shell systems <a class="reference internal" href="autodoc_glossary_options_c.html#term-reference-detci"><span class="xref std std-term">REFERENCE</span></a> is ROHF, <a class="reference internal" href="autodoc_glossary_options_c.html#term-wfn-detci"><span class="xref std std-term">WFN</span></a> is ZAPTN), DETCI will compute the ZAPTn series. <a class="reference internal" href="autodoc_glossary_options_c.html#term-guess-vector-detci"><span class="xref std std-term">GUESS_VECTOR</span></a> must be set to UNIT, <a class="reference internal" href="autodoc_glossary_options_c.html#term-hd-otf-detci"><span class="xref std std-term">HD_OTF</span></a> must be set to TRUE, and <a class="reference internal" href="autodoc_glossary_options_c.html#term-hd-avg-detci"><span class="xref std std-term">HD_AVG</span></a> must be set to orb_ener; these should happen by default for MPN = TRUE.</p>
<ul class="simple">
<li><strong>Type</strong>: <a class="reference internal" href="notes_c.html#op-c-boolean"><span>boolean</span></a></li>
<li><strong>Default</strong>: false</li>
</ul>
</div></blockquote>
<p>For larger computations, additional keywords may be required, as
described in the DETCI section of the Appendix <a class="reference internal" href="autodir_options_c/module__detci.html#apdx-detci"><span>DETCI</span></a>.</p>
</div>
</div>
<div class="section" id="arbitrary-order-perturbation-theory">
<span id="sec-arbpt"></span><span id="index-3"></span><h2>Arbitrary Order Perturbation Theory<a class="headerlink" href="#arbitrary-order-perturbation-theory" title="Permalink to this headline">¶</a></h2>
<p>The DETCI module is capable of computing energies for arbitrary
order Møller–Plesset perturbation theory (MPn, for closed-shell
systems with an RHF reference) and for Z-averaged perturbation theory
(ZAPTn, open-shell systems with an ROHF reference). However, please
note that these computations are essentially doing high-order CI (up to
full CI) computations to obtain these results, and hence they will only
be possible for very small systems (generally a dozen electrons or less).</p>
<p>The simplest way to run high-order perturbation theory computations is to
call, <em>e.g.</em>, <code class="docutils literal"><span class="pre">energy('mp10')</span></code> to invoke a MP10 computation or
<code class="docutils literal"><span class="pre">energy('zapt25')</span></code> to invoke a ZAPT25 computation. This will
automatically set several additional user options to their appropriate
values. The program uses the Wigner (2n+1) rule to obtain higher-order
energies from lower-order wavefunctions.</p>
<p>For the interested reader, the additional user options that are
automatically set up by the calls above are as follows. A call like
<code class="docutils literal"><span class="pre">energy('mp10')</span></code> sets <a class="reference internal" href="autodoc_glossary_options_c.html#term-mpn-detci"><span class="xref std std-term">MPN</span></a> to TRUE.
The program uses the Wigner (2n+1) rule by default
(<a class="reference internal" href="autodoc_glossary_options_c.html#term-mpn-wigner-detci"><span class="xref std std-term">MPN_WIGNER</span></a> = TRUE)
and figures out what order of wavefunction is
necessary to reach the desired order in the energy. The program then
sets <a class="reference internal" href="autodoc_glossary_options_c.html#term-max-num-vecs-detci"><span class="xref std std-term">MAX_NUM_VECS</span></a> to the required order in the
wavefunction.
By default, the requested n-th order energy is saved as the current
energy to the process environment. ZAPTN works essentially the same
way for an ROHF reference.</p>
</div>
<div class="section" id="arbitrary-order-coupled-cluster-theory">
<span id="index-4"></span><h2>Arbitrary Order Coupled-Cluster Theory<a class="headerlink" href="#arbitrary-order-coupled-cluster-theory" title="Permalink to this headline">¶</a></h2>
<p><em>This DETCI-based version of this feature is not yet released. However,
the current version of the code does include an interface to</em>
<a class="reference internal" href="mrcc.html#sec-mrcc"><span>Kallay’s MRCC</span></a> <em>code.</em></p>
<p>The DETCI module is also capable of computing arbitrary-order
coupled-cluster energies, using an approach similar to that of Hirata
and Bartlett <a class="reference internal" href="bibliography.html#hirata-2000-216" id="id4">[Hirata:2000:216]</a>, or of Olsen <a class="reference internal" href="bibliography.html#olsen-2000-7140" id="id5">[Olsen:2000:7140]</a>.
Notably, the approach in DETCI also allows arbitrary-order
<em>active space</em> coupled-cluster procedures. The general algorithm
for doing this in DETCI is inefficient compared to optimized
lower-order coupled-cluster codes and should not be used for CCSD,
where the CCENERGY module is much more efficient. For higher-order
CC (like CCSDT and beyond), the code is also not as efficient as the
MRCC code by Kállay, to which <span class="sc">Psi4</span> can interface (see Section
<a class="reference internal" href="mrcc.html#sec-mrcc"><span>Interface to MRCC by M. Kállay</span></a>); however, it may allow certain truncations of the model
space that might not be available presently in MRCC. For very small
systems, the code can be useful for testing of, for example, CCSDTQ or
its active-space CCSDtq analog <a class="reference internal" href="bibliography.html#piecuch-1999-6103" id="id6">[Piecuch:1999:6103]</a>.</p>
<p>To perform arbitrary-order coupled-cluster, set the DETCI
option <a class="reference internal" href="autodoc_glossary_options_c.html#term-cc-detci"><span class="xref std std-term">CC</span></a> to TRUE, and set
<a class="reference internal" href="autodoc_glossary_options_c.html#term-cc-ex-level-detci"><span class="xref std std-term">CC_EX_LEVEL</span></a> (note: not <a class="reference internal" href="autodoc_glossary_options_c.html#term-ex-level-detci"><span class="xref std std-term">EX_LEVEL</span></a>)
to the desired coupled-cluster excitation level, and invoke
<code class="docutils literal"><span class="pre">energy('detci')</span></code>. Various other DETCI options have a similar
option for coupled-cluster, usually named beginning with CC. The full
list of options is given in Appendix <a class="reference internal" href="autodir_options_c/module__detci.html#apdx-detci"><span>DETCI</span></a>.</p>
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<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">CI: Configuration Interaction</a><ul>
<li><a class="reference internal" href="#basic-detci-keywords">Basic DETCI Keywords</a><ul>
<li><a class="reference internal" href="#reference"><code class="docutils literal"><span class="pre">REFERENCE</span></code></a></li>
<li><a class="reference internal" href="#r-convergence"><code class="docutils literal"><span class="pre">R_CONVERGENCE</span></code></a></li>
<li><a class="reference internal" href="#ex-level"><code class="docutils literal"><span class="pre">EX_LEVEL</span></code></a></li>
<li><a class="reference internal" href="#fci"><code class="docutils literal"><span class="pre">FCI</span></code></a></li>
<li><a class="reference internal" href="#frozen-docc"><code class="docutils literal"><span class="pre">FROZEN_DOCC</span></code></a></li>
<li><a class="reference internal" href="#restricted-docc"><code class="docutils literal"><span class="pre">RESTRICTED_DOCC</span></code></a></li>
<li><a class="reference internal" href="#restricted-uocc"><code class="docutils literal"><span class="pre">RESTRICTED_UOCC</span></code></a></li>
<li><a class="reference internal" href="#frozen-uocc"><code class="docutils literal"><span class="pre">FROZEN_UOCC</span></code></a></li>
<li><a class="reference internal" href="#maxiter"><code class="docutils literal"><span class="pre">MAXITER</span></code></a></li>
<li><a class="reference internal" href="#num-roots"><code class="docutils literal"><span class="pre">NUM_ROOTS</span></code></a></li>
<li><a class="reference internal" href="#icore"><code class="docutils literal"><span class="pre">ICORE</span></code></a></li>
<li><a class="reference internal" href="#diag-method"><code class="docutils literal"><span class="pre">DIAG_METHOD</span></code></a></li>
<li><a class="reference internal" href="#opdm"><code class="docutils literal"><span class="pre">OPDM</span></code></a></li>
<li><a class="reference internal" href="#tdm"><code class="docutils literal"><span class="pre">TDM</span></code></a></li>
<li><a class="reference internal" href="#dipmom"><code class="docutils literal"><span class="pre">DIPMOM</span></code></a></li>
<li><a class="reference internal" href="#mpn"><code class="docutils literal"><span class="pre">MPN</span></code></a></li>
</ul>
</li>
<li><a class="reference internal" href="#arbitrary-order-perturbation-theory">Arbitrary Order Perturbation Theory</a></li>
<li><a class="reference internal" href="#arbitrary-order-coupled-cluster-theory">Arbitrary Order Coupled-Cluster Theory</a></li>
</ul>
</li>
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