Molcas Forum

philippt

Member

Registered: 2015-11-27

Posts: 2

active space orbitals are changed

Hi,

is it somehow possible to tell Molcas (we have version 8.0), that I don't want any other than the selected orbitals inside of my active space? Maybe some fancy DontChange keyword?
Molcas keeps changing those orbitals that I want against some other orbitals that seem to fit better for Molcas but which I absolutely don't want in my active space.

Thank you very much in advance
Philipp

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,012

Re: active space orbitals are changed

If you want the orbitals to optimized (i.e., if you want a true whatever-SCF result), some changes are bound to happen. If you get wrong orbitals in your active space, that can sometimes be avoided by changing the number of states in a state-average calculation, but it can also mean that the initial active space is not "correct" (there may be Rydberg orbitals trying to get in). You can try your calculation with a minimal basis set, and then expand it with EXPBAS (I think you need an ANO-type basis set for this).

If you really want your orbitals to remain unchanged, you can use the CIONly keyword in RASSCF, but then, well... your orbitals are not optimized.

Steven

Administrator

From: Lund

Registered: 2015-11-03

Posts: 95

Re: active space orbitals are changed

There are basically two ways to avoid unwanted rotations:

• freeze a number of inactive orbitals

• (ab)use the "supsym" keyword to put problematic orbitals in a separate subspace

The first way is only applicable for inactive orbitals. It freezes the orbitals up to a certain number per irrep. The second method was actually intended to handle more irreps than from Abelian groups, but it can be used to restrict rotations between any number of orbitals.

Even though there can be good reasons to do this, as mentioned above, you should be extra careful when using such restrictions. Aside from being careful yourself, you of course have to mention this when communicating your results, especially:

• why you did this: show that the problematic rotation is not relevant

• when in the procedure you started to use it: up to which point did you allow full freedom

Since you posted thid in the newbie corner, you probably will want to double check with a more experienced person in your group if what you are doing is sound.

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Always check the orbitals.