MCSCF GIAO

[ricaaardo]

Hello, everybody,
I want to make a CASSCF(6,6)-GIAO calculation for benzene, my problem is that I don't understand how to assign the active space, the main idea is that only pi orbitals are active orbitals(17, 20 and 21). Any help is welcome and thanks

[taylor]

From the fact that you speak of orbitals 17, 20, and 21 it would appear you are running without symmetry (presumably you have either turned symmetry off, with the Nosymmetry option, or you have the sort of geometry generated by many builder programs that is nearly-but-not-quite symmetric). My own view (obviously biased --- see the posting pinned to the top of the "Running Dalton" list...) is that this is not a good idea. But setting that aside for the moment, it is possible (but by no means guaranteed) that if you run a typical SCF/MP2/CASSCF calculation and specify 18 inactive orbitals, and 6 active orbitals with 6 active electrons, the calculation will converge to the result you want: a pi-electron CASSCF. If it does not yield the desired result, then you will have to start thinking about how to reorder orbitals, etc.

The alternative is to ensure you have a symmetric geometry to start with (for benzene this is a simple exercise in trigonometry), and you can then if you like specify the symmetry explicitly, or let the program determine it (either use the Generators keyword, or remove Nosymmetry). The program will use/recognize D2h, and the pi orbitals will appear in four of the symmetry species (which numbers they are or labels they acquire depends on molecular orientation) but they will be easy to recognize by looking for the basis functions that are perpendicular to the molecular plane. You can then specify explicitly which symmetry species have active orbitals, and how many they have.

There is also information about using symmetry in Chapter 24 of the Dalton manual.

Best regards
Pete

[hjaaj]

Symmetry or without symmetry, DO use MP2 to create initial orbitals for the MCSCF! This ia almost always a very good idea. In other cases, where you do not know which active space to choose, you can stop after MP2 and decide based on the MP2 natural orbital occupation numbers.