Molcas Forum

ABQTrap

Member

Registered: 2023-03-19

Posts: 21

occupation number of the active orbitals in casscf disordered?

Dear all,

I'm new to OpenMolcas. I performed the CASSCF calculations using small (6-31G) and medium basis sets (DEF-TZVP) while keeping the active space all the same. The types and shapes of the calculated active orbitals using both basis sets were almost identical. However, I found the occupation numbers of some active orbitals were significantly different if different basis sets were applied, which I can't understand what was wrong. The RASSCF.molden files were used to display the active orbitals and occupation numbers by Multiwfn software.

The active space had 8 electrons and 7 orbitals including two sigma orbitals, one pi orbital, one lone pair orbital. I tried to explore the bond dissociation process, therefore one sigma bond was almost breaking. When using the 6-31G basis set, the output occupation numbers were listed below:
sigma bond 1:  1.9737, 0.02664
sigma bond 2:  0.9997, 1.02336
pi bond:           1.92235, 0.0785
lone pair:         1.97548
these occupation numbers seem to be reasonable.

When the basis set changed to DEF-TZVP, the occupation numbers also changed a lot:
sigma bond 1:  1.97589, 0.02458
sigma bond 2:  1.00031, 0.07119
pi bond:           1.97717, 1.02174
lone pair:         1.92911
It seemed that the occupation numbers were disordered. How to solve this issue? Many Thanks.

input file

&GATEWAY
coord=triplet-scan-2.4.xyz  
basis=6-31G
&SEWARD
cholesky
&SCF               
charge=+1
zspin=0  
&RASSCF
LUMOrb
FileOrb= /path/to/triplet-2.4-guess.MP2Orb
nactel = 8
inactive= 31   
ras2 = 7        
spin = 3
OutOrbitals
Natural
1

Last edited by ABQTrap (2023-07-05 10:04:43)

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,012

Re: occupation number of the active orbitals in casscf disordered?

You have a sigma-sigma* state with 6-31G and a sigma-pi* state with DEF-TZVP. I guess the ordering depends on the basis set.

ABQTrap

Member

Registered: 2023-03-19

Posts: 21

Re: occupation number of the active orbitals in casscf disordered?

You have a sigma-sigma* state with 6-31G and a sigma-pi* state with DEF-TZVP. I guess the ordering depends on the basis set.

Thank you for your reply. I'm not quite familiar with the meaning of occupation number. So the sum of occupation numbers of two pi orbitrals (pi bonding and pi antibonding) is allowed to be much larger than 2.0 ? Because my calculation is ground state calculation, trying to describe the bond dissociation process.

Last edited by ABQTrap (2023-07-06 13:53:27)