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

[Peter Knowles]

Dear Tibor

You are right that the QVCCD equations are typically less well conditioned than CCSD, and Kirk’s suggestion is just the one I’d try first.  If you were to expose an entire input, I’d be happy to work with you to see if one can do better.

Best regards,
Peter 

> On 22 May 2017, at 09:56, 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>_______________________________________________
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--
Prof. Peter J. Knowles             
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
Telephone: +44 29 208 79182
Email: KnowlesPJ at Cardiff.ac.uk 

Yr Athro Peter J. Knowles
Yr Ysgol Cemeg, Prifysgol Caerdydd,
Prif Adeilad, Plas y Parc
Caerdydd CF10 3AT, DU
Ffôn: +44 2920 879182
Ebost: KnowlesPJ at Caerdydd.ac.uk
Ysgrifennwch ataf yn Gymraeg neu Saesneg