CCL: Can CASSCF energy be a descriptor for the stability of active spac

 Sent to CCL by: "Nuno A. G. Bandeira"
 [nuno.bandeira++tecnico.ulisboa.pt]
 [Resent to CCL by the Admin]
 Having a balanced active space is not an easy thing to achieve. I find that
 often its a tedious trial and error procedure to get to know how the algorithm
 behaves. A robust second order algorithm will usually get to the best solution
 but it always depends on the number of roots you ask for.
 First: is CASSCF the end game? Will you perturb the wavefunction afterwards? If
 so then it wont matter so much to have a large AS.
 For different spin state it might be more important to correlate other orbitals
 than the ones youre interested in. Theres nothing you can do about that, but you
 can try to add more orbitals into the active space until you have a matching AS
 across all spin states.
 I would start by looking at the MP2 natural orbitals at the frontier region to
 see which orbitals of your ligand are more relevant. You might not need that
 many but you need a matching set for every spin state. You may not even need all
 the 3ds it just depends on the electronic structure of the complex itself.
 Hope this helps.
 All the best,
 Nuno
 -----------
 Nuno A. G. Bandeira, AMRSC
 Email: nuno.bandeira*o*ciencias.ulisboa.pt
 BioISI - Biosystems & Integrative Sciences Institute;
 8.5.53 - C8, Faculty of Sciences, University of Lisbon
 Campo Grande- 1749-016 Lisboa Portugal
 Phone: +351 21 750 08 45
 --------
 https://www.researchgate.net/profile/Nuno_Bandeira
 http://www.researcherid.com/rid/B-6399-2012
 http://orcid.org/0000-0002-5754-7328
 http://www.cienciavitae.pt/pt/4A19-ACC3-E225
 http://pt.linkedin.com/pub/nuno-a-g-bandeira/47/55a/2aa
 > From: Muhammed mbtemiz3:-:gmail.com
 Sent: 20 August 2021 18:05
 To: Bandeira, Nuno A. G.
 Subject: CCL: Can CASSCF energy be a descriptor for the stability of active
 space?
 Dear CCLers,
 I am trying to construct an active space (AS) for a Co=O transition metal
 complex. I am interested in the metal d orbitals, oxygen 2p orbitals and one
 specific ligand orbital. I have managed to get the orbitals I desire for three
 spin states (doublet, quartet, sextet) after a couple of orbital rotations and
 got the energies with a relatively good basis set. (This is the first AS)
 However while doing the orbital rotations, I found a lower energy solution for
 the doublet state with the active space of 5 d orbitals, 3 Oxygen 2p orbitals,
 and 1 Oxygen 2s orbital - ligand orbital is swapped with Oxygen 2s orbital (This
 is the second AS). This AS is no good since I am also interested in the ligand
 itself, however the energies made me question the stability of the first active
 space.
 Now the problem is this: doublet has two solutions; first AS and second AS.
 Second AS is around 30 kcal/mol lower in energy than the first AS, however it
 does not include the orbital I am interested in.
 My question is this, do these results tell anything about the nature of the
 active space?
 Should I lean towards the second AS since it has a lower energy solution than
 the first one?
 My reasoning is this;
 1- The lower energy active space tells me that the initial AS is not very well
 balanced and additional orbitals might be included in the AS. However this
 approach didn't solve my problem - got similar results even with the highest
 electron/orbital pair I technically can run. (including double shell orbitals
 are also not rewarding). And without the ligand orbitals, this approach hit a
 wall.
 2- These solutions live in different spaces and we can't compare them. This is
 similar to constructing two different active spaces with different
 electron/orbital pairs and trying to compare their energies - i.e comparing
 CAS(10,10) energy with CAS(12,12).  Although both solutions have equal number of
 electrons/orbitals in this case, the nature of the wavefunction is different for
 both. First solution includes configurations with excitations from the ligand
 orbitals, however the second one considers the excitations from oxygen 2s
 orbital. Therefore I can continue to work with the first active space without
 worrying about the second AS solution.
 Although I am not sure about the last point, I would appreciate if someone
 guides me to the right direction here.
 Thank you.
 Muhammed.
 *I am aware of the double shell effect but ignored it here for the sake of
 simplicity.
 --
 Muhammed Buyuktemiz
 Chemistry Department, Gazi University