Curently, Iím experiencing some difficulties with CASSCF calculations on a certain molecule, which I donít know how to overcome. The problem is that I would like to create an active space which incorporates the moleculeís pi-system, but that I canít 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 donít make a whole lot of sense.
In order to get the active space right, Iíve tried a few things. Iíve 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, Iíve 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 thatís giving me problems, I managed to get correct CASSCF orbitals when I leave the two p lone pairs out of the active space.
Iím not sure whatís 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!
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[ CASSCF problem.rar ] Input and output files