Molcas Forum

juno

Member

Registered: 2019-03-13

Posts: 11

cassscf input for an open-shell molecule

Dear All,

I am new to MOLCAS software. I would like to perform a casscf calculation on an open-shell organic molecule.
The example of benzene is given in sample, but that is a closed shell system and the input orbitals are from RHF based calculations.
I aim to calculate singlet-triplet gap in organic diradical molecule, but I am not able to form ROHF orbitals for the system to be given as input.
I am not able to write a CASSCF input for such a case.
Can anyone guide me ?

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,079

Re: cassscf input for an open-shell molecule

All the active orbitals are potentially an "open shell", because the electrons are not required to be paired in them. There's nothing special in the CASSCF input for a diradical, just make sure the number of electrons and the active orbitals are right. If the diradical is the ground state, you should get it, if it's not, you'll have to compute excited states (CIRoot).

For initial orbitals, you don't need ROHF orbitals, you could use RHF ones, or UHF orbitals (from singlet or triplet), or orbitals from a cation, or MP2 canonical orbitals, or localized orbitals... Any kind of initial orbitals is good, as long as you select the "correct" active space, because the CASSCF procedure will optimize the orbitals from there.