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

[Daniel Kats]

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.,
file,2,molecule.wfu
.....
hf
{oqvccd(t),shiftp=0.8,shifts=0.6;maxit,100;start,5200.2;save,5200.2}

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:
{oqvccd(t),shiftp=0.8,shifts=0.6,nomp2=1;maxit,100}

3. Another option is to use e.g. distinguishable cluster amplitudes as a
starting guess for OQVCCD:
{bdcd,nomp2=1,shiftp=0.8,shifts=0.8;maxit,100;save,5200.2}
{oqvccd(t);maxit,100;start,5200.2}

You can find outputs of these last two options attached.

Best wishes,
Daniel

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
>> >
>> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.molpro.net_mailman_listinfo_molpro-2Duser&d=DwICAg&c=C3yme8gMkxg_ihJNXS06ZyWk4EJm8LdrrvxQb-Je7sw&r=mqaXWsZ0ghQR-pXvPLjY_g&m=OtWbWWEZBfJvvNsgh34ZzXKGiVsLFXfovJUhURi27qM&s=M6yVxvlkDsAB_09PjJGiboroHxeAxsy3-OuOSeigYfk&e=
>>
>>
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-- 
Daniel Kats
Institute for Theoretical Chemistry
University of Stuttgart
Pfaffenwaldring 55
D-70569 Stuttgart
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