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Thomas
thomaspijper@hotmail.com

Curently, I’m experiencing some difficulties with CASSCF calculations on a certain molecule, which I don’t know how to overcome. The problem is that I would like to create an active space which incorporates the molecule’s pi-system, but that I can’t seem to get the right orbitals into the active space.

The molecule is a 20 atom organic compound which has four pi bonds, and two p lone pairs located on two sulphur atoms, all of which are in conjugation. So, I opted for a 12 electrons, 10 orbitals active space. Starting from Pipek-Mezey localized orbitals, a first CASSCF calculation converged, but visualization of the orbitals showed that they don’t make a whole lot of sense.

In order to get the active space right, I’ve tried a few things. I’ve tried the FULLNR converger, without much success. I also tried a state-averaged optimization instead of a groundstate-specific one, with the same results. Finally, I’ve used NBO localized orbitals as the starting orbitals, an approach which has never failed me before, but again I got the wrong active space.

With a very similar compound, an active space of the same size could be obtained without a problem. Furthermore, for the molecule that’s giving me problems, I managed to get correct CASSCF orbitals when I leave the two p lone pairs out of the active space.

I’m not sure what’s going wrong here, so I would like to ask if someone has an idea and can perhaps help me get the right orbitals into the active space. I have provided an input file as well as an output file with the wrong orbitals with this message. Thanks in advance!


Kind regards,
Thom

This message contains the 505 kb attachment [ CASSCF problem.rar ] Input and output files