Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly

Alex Granovsky
gran@classic.chem.msu.su

>Great! Is it possible to obtain 8.0.0 beta already?

Latest betas are available from us upon request.
Please drop me a letter if you are interested.

>>> As for the check of the overlap matrix of non-orthogonal eigenvectors, it's a good idea, thanks. So, if off-diagonal values are small and all eigenvalues of non-symmetric H_eff are real, it's no need to further increase H_eff, isn't it?
>>No, the idea is that if they are large and/or some eigenvalues
>>are complex, your XMCQDPT2 job is definitely running into problems.  

>What off-diagonal value should be considered as large: ~0.1, or ~0.01, or ~1?

I'd suggest to avoid off-diagonals larger than ca. 0.1-0.2
and safely ignore smaller values. Very large off-diagonal values
of overlap of two XMCQDPT2 states typically mean that these two
states attempt to describe the same real state. The most
likely reason of this is the poor choice of the model space
(e.g., due to some low-lying spurious CASSCF states entering
model space). Another reason could be intruder state problem.

> and is it safe if the imaginary part of an eigenvalue of non-symmetric H_eff is ~1.0e-16?

It is absolutely safe. These small values are artifacts of
accumulation of roundoff errors inside non-symmetric matrix
diagonalization code. Look at the output and you'll find that
complex eigenvalues are always coming in pairs of conjugated
values.

> And what should I do in case of problems? change some XMCQDPT
> parameters or start with another CASSCF solution?

In most cases this means you need to double-check CAS active
space and CASSCF solutions.

>>>By the way, in the next FF version, is it possible to print the WARNING that off-diagonal values are large or some eigenvalues of non-symmetric H_eff are complex?
>>Is it too difficult to check this manually in the output? :)

>For me, it is not difficult, but there are many less experienced FF users who knows nothing about possible problems. And I learned about this issue just now, in this discussion.

Well, it may follow from your paper in J. Chem. Phys., but it is still in press and not available...

The paper will be available online in a week or so.

This problem is not discussed in the paper. Actually, these are the
well-known facts for a (very) small audience. :)

Cheers,
Alex