Molcas Forum

Anugraha

Member

Registered: 2025-12-17

Posts: 22

Sigma star orbital moves out of active space

Dear Molcas Developers,
I am performing a CASPT2 calculation on ground state of my molecule (Cs symmetry).

& SEWARD 
& RASSCF 
 Fileorb = Project.ScfOrb
 symmetry = 1
 Iterations = 200 100
 Spin = 1
 nActEl = 10 0 0
 Inactive = 25 0
 Ras2 = 1 8
 ciroot = 1 1 1
& CASPT2
 Multistate 
 1 1
 IPEAshift = 0.0

In the active space i have included 5 pi orbitals, 3 pi star orbital and 1 sigma star orbital. Before calculation i edited the .scforb file and my intended sigma star orbital was number  26 in the active space.
But after the calculation i have checked the orbitals and the 26th orbital has no longer the character of sigma star orbital.Instead the 27th orbital which was not in the active space has the sigma star character.

I have also tried increasing the number of CIroots and the same problem persits.

Is there any way to include this sigma satr orbital in the active space?

Or shall i remove this orbital from the active space?

I would be thankful if anyone can comment about this problem.

Thank you
Anugraha

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,203

Re: Sigma star orbital moves out of active space

If an orbital rotates out of the active space, it's probably because its occupation does not differ significantly from 0 or 2 in the state(s) included. Increasing the number of roots in the state average can fix it if you go high enough to include some state where electrons are excited to/from the desired orbital. But whether or not that's adequate, depends on your system. Also, typically if you include a sigma star you also want to include the corresponding sigma.