I am using CASSCF method by qchem software. So, i am facing difficulty in how to choose active orbitals and active electrons for a molecule and also i am also confused about the step wise procedure to optimse molecule using CASSCF method.
Hi Aarzoo,
your question is a little bit vague, so apologies if my answer is off the mark. The basic way to set up the active space, in most of CASSCF codes, just requires as input from the user i) the number of active orbitals (keyword CAS_N_ELEC, in qchem) ii) the number of active electrons (keyword CAS_N_ORB, in qchem). The active orbitals are then automatically selected in order to fulfil those two requirements. Example: if you have a closed shell reference, asking a [4,5] active space (that is: 4 active electrons, 5 active orbitals) will select an active space ranging from HOMO-1 to LUMO+2. The active orbitals are taken as a continuous block around the HOMO-LUMO orbitals. A previous re-ordering of the orbitals list is often necessary, in order to include some specific orbitals into the active space. Some codes (eg. Molcas) also offer more sophisticated ways to select the active space, but they are intended for experienced users.
Hi Dr.
I suggest for you this site
https://pubs.acs.org/doi/10.1021/acs.jctc.9b01255#:~:text=The%20complete%20active%20space%20self,growth%20in%20the%20computational%20cost.
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