CAS-SCF: state-averaged and state selection

Hello Q-Chem community,

I am performing CAS-SCF calculations and I have two main questions that are not explained in the Q-Chem tutorial:

  • List item By indicating multiple roots, a state-averaged CAS-SCF calculation performed? If so, can the root weights be manually adjusted?

  • List item With “CAS_N_ORB” command the number of active space orbitals is specified, but how can the orbitals in the active space be selected manually? If this is not possible, how does the program the selection?

Thanks in advance for your time and help.



Same question was asked here (State-Average CASSCF) but not yet answered, will have to wait for someone more expert than myself.

Hi Antonio. I’ve been testing CASSCF implementation on Q-Chem 5.4. It seems the SA-CASSCF is not implemented. What the program does is perform a CASSCF calculation for the ground state, then it computes the energy for the requested roots using CASCI method.

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Hello all,

I can confirm what Arturo says. As far as I’m aware, SA-CASSCF is not yet implemented; we are happy to submit a feature request for this in Q-Chem’s internal system.

Regarding your second question, for CAS-CI jobs in the RASCI1 module, you can specify a user-defined RAS2 space using RAS_ACT_ORB; were you looking for a functionality similar to this? If so, I don’t believe you can manually select orbitals for the active space in CAS-SCF jobs, unfortunately. The orbitals are selected automatically based on CAS_N_ORB and CAS_N_ELEC; ex. in the case of the nitrogen dimer, if you set CAS_N_ORB=6 and CAS_N_ELEC=6, using the singlet state as the reference, it will do CAS-SCF(6,6) using an active space that consists of the 3 highest-energy occupied and 3 lowest-energy virtual orbitals.

Let us know if this doesn’t answer your questions!

Best regards

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Hi Arturo. Thank you very much for the information!

Hi Shannon. This answers my question perfectly. Thank you very much for your detailed explanation!

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