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

[Tibor Győri]

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
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