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<LI><A NAME="tex2html720"
  HREF="node27.html#SECTION00062010000000000000">5.2.0.1 Structural optimization</A>
<LI><A NAME="tex2html721"
  HREF="node27.html#SECTION00062020000000000000">5.2.0.2 Molecular Dynamics</A>
<LI><A NAME="tex2html722"
  HREF="node27.html#SECTION00062030000000000000">5.2.0.3 Free-energy surface calculations</A>
<LI><A NAME="tex2html723"
  HREF="node27.html#SECTION00062040000000000000">5.2.0.4 Variable-cell optimization</A>
<LI><A NAME="tex2html724"
  HREF="node27.html#SECTION00062050000000000000">5.2.0.5 Variable-cell molecular dynamics</A>
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<H2><A NAME="SECTION00062000000000000000">
5.2 Optimization and dynamics</A>
</H2>

<P>

<H4><A NAME="SECTION00062010000000000000">
5.2.0.1 Structural optimization</A>
</H4>
For fixed-cell optimization, specify <TT>calculation='relax'</TT> and 
add namelist &amp;IONS. All options for a single SCF calculation apply, 
plus a few others. You
may follow a structural optimization with a non-SCF band-structure
calculation (since v.4.1, you do not need any longer to update the 
atomic positions in the input file for non scf calculation).
<BR>
See Example 03.

<P>

<H4><A NAME="SECTION00062020000000000000">
5.2.0.2 Molecular Dynamics</A>
</H4> 
Specify <TT>calculation='md'</TT>, the time step <TT>dt</TT>, and possibly the number of MD stops <TT>nstep</TT>.
Use variable <TT>ion_dynamics</TT> in namelist &amp;IONS for a fine-grained control
of the kind of dynamics. Other options for setting the initial
temperature and for thermalization using velocity rescaling are
available. Remember: this is MD on the electronic ground state, not
Car-Parrinello MD.
See Example 04.

<P>

<H4><A NAME="SECTION00062030000000000000">
5.2.0.3 Free-energy surface calculations</A>
</H4>

<P>
Once <TT>PWscf</TT> is patched with the <TT>PLUMED</TT> plug-in, it is possible to 
use most PLUMED functionalities by running <TT>PWscf</TT> as: 
<TT>./pw.x -plumed</TT> plus the other usual <TT>PWscf</TT> arguments.
The input file for <TT>PLUMED</TT> must be found in the specified 
<TT>outdir</TT> with fixed name <TT>plumed.dat</TT>.

<P>

<H4><A NAME="SECTION00062040000000000000">
5.2.0.4 Variable-cell optimization</A>
</H4>

<P>
Since v.4.2 the newer BFGS algorithm covers the case of variable-cell
optimization as well. Note however that variable-cell calculations
(both optimization and dynamics) are performed with plane waves and 
G-vectors <EM>calculated for the starting cell</EM>. This means that if 
you re-run a self-consistent calculation for the final cell and atomic 
positions using the same cutoff <TT>ecutwfc</TT> (and/or <TT>ecutrho</TT> 
if applicable), you may not find exactly the same results, unless your
final and initial cells are very similar, or unless your cutoff(s) are very
high. In order to provide a further check, a last step is performed in
which a scf calculation is performed for the converged structure, with
plane waves and G-vectors <EM>calculated for the final cell</EM>. Small 
differences between the two last steps are thus to be expected and give
an estimate of the reliability of the variable-cell optimization.
If you get a large difference, you are likely quite far from convergence
in the plane-wave basis set and you need to increase the cutoff(s).

<P>

<H4><A NAME="SECTION00062050000000000000">
5.2.0.5 Variable-cell molecular dynamics</A>
</H4>

<P>
"A common mistake many new users make is to set the time step <TT>dt</TT>
improperly to the same order of magnitude as for CP algorithm, or
not setting <TT>dt</TT> at all. This will produce a ``not evolving dynamics''.
Good values for the original RMW (RM Wentzcovitch) dynamics are 
<TT>dt</TT> <!-- MATH
 $= 50 \div 70$
 -->
= 50 &#247; 70. The choice of the cell mass is a delicate matter. An
off-optimal mass will make convergence slower. Too small masses, as
well as too long time steps, can make the algorithm unstable. A good
cell mass will make the oscillation times for internal degrees of
freedom comparable to cell degrees of freedom in non-damped
Variable-Cell MD. Test calculations are advisable before extensive
calculation. I have tested the damping algorithm that I have developed
and it has worked well so far. It allows for a much longer time step
(dt=<!-- MATH
 $100 \div 150$
 -->
100 &#247; 150) than the RMW one and is much more stable with very
small cell masses, which is useful when the cell shape, not the
internal degrees of freedom, is far out of equilibrium. It also
converges in a smaller number of steps than RMW." (Info from Cesar Da
Silva: the new damping algorithm is the default since v. 3.1).

<P>
See also <TT>examples/VCSexample</TT>.

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