I am doing some test CASSCF calculation followed by a NBO analysis in Gaussian09. For ex: If I want to analyze the triplet spin of the first excited B2 state (in c2v symmetry) of water, how do I write my command line input?
I am also interested to know how one can define the active space in the input?
For ex:
Closed orbitals: 2a1,0b1,0b2,1a2
Occupied orbitals: 4a1,2b1,2b2,1a2
Or it is much easier to do in GAMESS package?
Thanks in advance
Dear Somnath,
Within my knowledge, it is difficult to control active space of CASSCF calculation by symmetry in Gaussian program suite. GAMESS provides such a function, but, it is better to use MOLPRO or MOLCAS for this kind of matter, since calculation converger in these programs are more robust than the one in GAMESS.
Dear Professor Mori,
Thank you for your kind response. I am extensively using Molpro package for ab initio calculation. The only reason I am trying other platform is because I need NBO analysis for ground and some specific excited states as Molpro does not offer NBO analysis while keeping symmetry (such as C2v). I have also tried on GAMESS (free version), without any luck yet.
You first need to identify the symmetry/numbering of the orbitals you would like in your active space - try running an initial HF calculation with Pop=Reg. By default Gaussian will include enough occupied orbitals to provide the number of electrons you ask for in CASSCF, and enough virtuals to make up the total number of orbitals you request. For example, in your input file you would request CASSCF(11,6) for 11 electrons in 6 orbitals.
If you are lucky, this will automatically lead to the active space you want.
If not, you need to use the Guess=Alter command to interchange orbitals. There is a good example in the Gaussian manual of how to do this.
http://gaussian.com/g_tech/g_ur/k_casscf.htm
Dear Professor Hill,
Thank you very much indeed. It has partly solved the problem.
Dear Fellows,
Could anyone please tell me the correct syntax for using CASSCF MP2 in Gaussian?
I got the first (#N CASSCF(4,4)/6-311+G* Guess=Alter Pop=Reg) done correctly
I want to do it with some higher level for which I tried (#N CASMP2(4,4)/6-311+G* Guess=Alter Pop=Reg) and some other variations of this but always got Syntax error.
Try CASSCF and MP2 separated. Like, CASSCF(4,4)/6-311+G* MP2. I hope that helps.
Can someone answer this specific line of the question "If I want to analyze the triplet spin of the first excited B2 state (in c2v symmetry) of water, how do I write my command line input?" .
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