[CPMD-list] geometry optimization - isotopes and convergence doubt

[Axel Kohlmeyer]

On Tue, 1 Apr 2008, Mark Kosmowski wrote:


dear mark,

MK> AK> by themselves do not provide. i.d. using DFT, pseudpotentials
MK> AK> and finite (fft compatible) grids.
MK> AK>
MK> 
MK> Right - our group compares theoretical results to experimental result
MK> to discuss the accuracy of current calculations.

well, i'd like to remark that for the kind of systems you seem to be
running, this is a bit unfair w.r.t. the calculations. plane-wave 
pseudopotential calculations are best suited for (dynamical) bulk 
systems, where you benefit most from good parallelization, lack 
of BSSEs and no linear dependency problems, that programs with gaussian 
based atom centered basis sets are facing. 

for isolated systems, those problems are much less pronounced and
with CPMD you have to spend a lot of additional effort to compute 
"vacuum" and to decouple the interactions between periodic images.

MK> AK> optimization which in turn depends on the (final) convergence for the
MK> AK> wavefunction.
MK> AK>
MK> 
MK> I am using the default convergence parameters for wave function and
MK> geometry.  (I watch for 10-4 in the geometry steps to know how close
MK> the run is to being done.)

the default convergence in CPMD is chosen to give a reasonable starting
point for MD runs. since you are using (excessively?) high cutoffs
and you want to have consistent and "accurate" results, you have to
crank up the convergence on the wavefunction, or else you won't have
accurate forces. in CP-dynamics it does not matter how accurate your
starting forces are since you are oscillating around the ground state
anyways. for BO-dynamics already you'll have to go to _much_ tighter
convergence (which can be relaxed a bit again, when using wavefunction
extrapolation) to conserve energy.

the major improvement that you gain from the high cutoff is the
reduced "ripple-effect", i.e. artefacts in the potential from 
finite number of plane waves in combination with the spline 
interpolation on the grid. the rule of the thumb is, that you need
to converge a wavefunction at least two orders of magnitude tighter
than the forces. i.e. you'd need 1.e-6 convergence when your after
1.e-4 accuracy in the forces.

MK> The only difference in the inputs was the isotope definition.  I have
MK> a bash script that runs the deuterated case first, then copies to a
MK> new directory and runs a sed script to change the ISOTOPE section from
MK> 2H to 1H.  The current run is with light main group elements using LDA
MK> DFT and pade pseudopotentials from the Goedecker directory (downloaded
MK> from the CPMD website).

hmm... that reminds me of another potential source of error:
you are treating the hydrogen and deuterium atom cores both as 
classical particles. particularly for hydrogen that may not be
sufficient and introduce additional systematic errors beyond
the numerical accuracy.

MK> Once the LDA work is done, I plan on also using at least the PBE
MK> functional and associated pseudopotentials.  Are the pseudopotials of
MK> different functionals different enough to require a full convergence
MK> study by cutoff and k points as I am already doing for LDA / pade?

convergence behavior should be similar.

[...]

MK>      MK> I am assuming that this just means that 50 Ry is not a high enough
MK>      MK> cutoff energy for my system regardless what the parameter vs. cutoff
MK> AK>
MK> AK> that is more likely because of an instability in the lanzcos
MK> AK> procedure and you'll have to tweak the lanzcos related parameters
MK> AK> to get it to run more stable. it looks also like the code detected
MK> AK> that and resetted the diagonalization.
MK> 
MK> Thank you for explaining this.

BTW: these calculations should become much faster with the next 
CPMD release as alessandro curioni and his group have extended k-point
support for direct wavefuntion optimization (e.g. via ODIIS instead 
of LANCZOS) to geometry optimizations and BO-MD. 

cheers,
   axel.


-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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