[molpro-user] about uccsd(t)
Gerald Knizia
knizia at theochem.uni-stuttgart.de
Tue Jan 1 16:34:14 GMT 2008
On Tuesday 01 January 2008 16:46, rachel.crespo at uam.es wrote:
> tried to do some uccsd(t) and program shows the following error:
>
> ?CONVERGENCE NOT REACHED AFTER MAX. ITERATIONS
> ? Error
> ? No convergence
> ? The problem occurs in uccsd
>
> I tried to change the number of iteractions but I can do it.
You can set the maximal number of iterations like
{uccsd(t); maxit,137}
but most likely this is not your main problem here, as RHF-UCCSD(T) really is
supposed to converge in at max 25 iterations except in the most pathological
of cases or if something different is wrong.
Typical things to look out for:
o Is the geometry specification correct? Molpro outputs bond lengths and
bond angles in the beginning. You should check if they are reasonable.
Especially look for Bohr/Angstrom conversion issues.
o Is the state being optimized reasonable? (charge, spin, symmetry and so
on).
o Are the chosen basis sets the ones you intended? E.g., if specifying basis
sets manually, did you provide functions for each required angular momentum?
Other things to consider:
o If using very large or very diffuse basis sets (e.g., AV5Z and larger),
you might need to increase the integral accuracy in order to reach high
convergence thresholds:
gthresh,oneint=1e-15
gthresh,twoint=1e-14
> In some cases the calculation stop with the same message but the energies
> seem to be converged.
In the iteration log two of the outputs are VAR(P) and VAR(S). These specify
how the excitation amplitude vectors changed in the current iteration and are
rather direct measures of the residual size, i.e., of how closely the current
amplidues solve the uccsd equations they are supposed to solve. If these
don't tend to zero, the equations are not solved and you cannot trust the
energies obtained, even if they _seem_ converged.
That being said, the output you posted looks rather puzzling. I'd assume
something like that could only happen if uccsd is jumping between two
different fixed points in the iterations. Could you provide the input this
originates from?
--
Gerald Knizia
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