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

[Peterson, Kirk]

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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