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Ole_
hueter@phc.uni-kiel.de

I'm trying to calculate XMCQDPT2 energies of Pentafluorobenzene along one of its normal modes. The CAS consists of 7 electrons in 6 orbitals, and I'm averaging the lowest 7 states. The resulting energies (MCSCF and XMCQDPT2) in the Franck-Condon structure are very reasonable, and convergence seems good to me.

When I'm beginning to distort the molecule, the MCSCF potential energy curves of the states are smooth and reasonable. The XMCQDPT2 energies, however, start to jump around and show what I would call unphysical values. At larger distortions, the XMCQDPT2 energies eventually become smooth.

I included a zip file which has selected input and output files to keep the file size small, as well as plots of the MCSCF and XMCQDPT2 energies to make this more clear (plotted is energy in eV over normal coordinate Q). From the plots, the problem is obvious, I hope. The output of the XMCQDPT2 program seems to be ok (eigenvectors etc), I checked all the points given in the documentation.

I cannot figure out whats causing this behaviour. I guess it is not a "physical" problem with the MOs or anything, but rather an unfortunate combination of incompatible keywords, because the MCSCF energies are fine. But the XMCQDPT2 step does not require that many stettings... any help would be really appreciated here!

I'm using the latest Firefly 8.2.0.

Thank you so much for your help!
Best regards, Ole

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