[molpro-user] OQVCCD does not converge at extreme geometries, but CCSD does

Daniel Kats dnkats at gmail.com
Tue May 23 12:25:37 CEST 2017

Dear Tibor,

There are few things one can try:

1. The best solution would be to go from a single-reference geometry to the
multi-reference geometries in small steps, always restarting from the
previous calculation, e.g.,

2. If it's not possible, one can try to generate a more reasonable starting
guess than MP2.
First option is to simply use a zero-vector instead of MP2-amplitudes:

3. Another option is to use e.g. distinguishable cluster amplitudes as a
starting guess for OQVCCD:

You can find outputs of these last two options attached.

Best wishes,

On Tue, May 23, 2017 at 9:01 AM Tibor Győri <tiborgyri at gmail.com> wrote:

> Yes, I have tried {oqvccd(t);shift,1,1} and
> {oqvccd(t),shifts=0.2,shiftp=0.2,thrdis=1.0;diis,1,1,15,1;maxit,100;}
> Since I only tried these at a single geometry, it is possible that they
> would help at the other problematic geometries, but they are no silver
> bullet.
> 2017-05-22 7:42 GMT+02:00 Peterson, Kirk <kipeters at wsu.edu>:
>> Dear Tibor,
>> so you tried shifting both the singles and pairs via something like:
>>  {oqvccd(t);shift,1,1}   ?
>> best,
>> -Kirk
>> > On May 21, 2017, at 5:54 AM, Tibor Győri <tiborgyri at gmail.com> wrote:
>> >
>> > Dear Molpro users,
>> >
>> > Recently I have run into some issues while trying to use OQVCCD(T) for
>> the purpose of calculating energies for PES fitting. The problem is, that
>> to fit a global PES some calculations have to be run at pretty extreme
>> geometries, and I have seen many examples, where ordinary CCSD and CCSD-F12
>> converges but OQVCCD does not.
>> >
>> > The awesome multiple bond breaking characteristics of OQVCCD(T) would
>> be really advantageous for fitting a global PES, but these convergence
>> issues are hindering my efforts. I have tried to increase the maximum
>> number of iterations allowed to 100, but a number of calculations still do
>> not converge.
>> > I also tried to use level shifts and increased DIIS size at one
>> geometry, but it did not work any better.
>> >
>> > Is there anything I could do to increase the chance of OQVCCD
>> converging in difficult cases? Since these input files are automatically
>> generated, anything that requires manual per-input tuning is pretty much
>> out of the question.
>> > I have attached an abridged log file of a calculation that had issues.
>> >
>> > Best wishes,
>> > Tibor Győri
>> > MSc student
>> > University of Szeged
>> >
>> > <oqvccd.log>_______________________________________________
>> > Molpro-user mailing list
>> > Molpro-user at molpro.net
>> >
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Daniel Kats
Institute for Theoretical Chemistry
University of Stuttgart
Pfaffenwaldring 55
D-70569 Stuttgart
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