Molcas Forum

Lukhmanul Hakeem k

Member

Registered: 2024-02-05

Posts: 45

Clarification on CASSCF Active Space and Energy Differenc

Dear Sir,

I have performed the CASSCF calculation in two ways:

In the first case, I carried out an SCF calculation separately and then selected an active space of (12,12). I edited the ScfOrb file by marking the desired orbitals with “2” and placed it in the scratch directory as INPORB. The corresponding RASSCF energy obtained was –761.394.

In the second case, I performed the CASSCF calculation in a single step (SCF followed by CASSCF (12,12)) without editing the ScfOrb file. The energy obtained in this case was –761.397.

I would like to confirm whether the second calculation represents the correct active space, since it gives a slightly lower energy.

Kind regards,
Lukhmanul Hakeem K.

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,165

Re: Clarification on CASSCF Active Space and Energy Differenc

There is no such thing as "correct active space". What is correct depends on what you want to achieve obtain. The best active space for studying low-lying electron excitations is probably different from the ones for studying core ionizations, or bond-breaking reactions, or intermolecular interactions... If your only goal is to get the lowest possible energy, then yes, the second is "better". But you should have a prior idea of what the active space should look like (which orbitals are included, which orbitals are not), and then you can judge based on that.

Lukhmanul Hakeem k

Member

Registered: 2024-02-05

Posts: 45

Re: Clarification on CASSCF Active Space and Energy Differenc

Dear Sir,
Thank you for your response.

Kind regards,
Lukhmanul Hakeem K.