Molcas Forum

ABQTrap

Member

Registered: 2023-03-19

Posts: 24

state-specific CASSCF opt structure rely on state number of SA(CIROOT)

Dear all,

  I am new to OpenMolcas. I tried to perform the geometrical optimization of ground-state by state-specific CASSCF method using state-average (SA) CASSCF wavefunction. The reason I used state-average  CASSCF wavefunction is I needed to add one low-lying lone pair orbital into the active space. If only ground-state wavefunction was used, this lone pair orbital would be removed out the active space by OpenMolcas automatically due to the occupation number closed to 2. Below is the input file, and keyword rlxroot was used to assign the ground state for optimization.

&GATEWAY
 coord=reac.xyz
 basis=DEF2-TZVPP

>>> COPY $MOLCAS/CAS1210/reac-cas1210.RasOrb INPORB

>>> Do While  

&SEWARD

&RASSCF
 LUMOrb
 nactel = 12 0 0
 inactive= 29   
 ras2 = 10        
 spin = 1
 ciroot = 3 3 1
 rlxroot = 1

&MCLR

&ALASKA

&SLAPAF
 MAXStep = 0.1

>>> EndDo

  My question is how many states should I included for state-average (SA) CASSCF wavefunction? I found different number of states of SA-CASSCF wavefunction may influence the optimized structure obtained. For example, the bond length may vary by almost 0.07 angstrom between ciroot = 3 3 1 and ciroot = 4 4 1.  I suppose the keyword ciroot should not have such huge impact on the optimized structure since keyword rlxroot was kept the same. So how should I correctly choose the suitable value for ciroot? Or, If I need to consider the weights between different states? Thanks a lot.

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,085

Re: state-specific CASSCF opt structure rely on state number of SA(CIROOT)

The number of states affects the results, because the orbitals are optimized for a different average of states. It's a bit like the active space, there's no "right" answer, it depends on your system and what you're studying. In general, you should try to have all states well balanced, and I don't recommend changing the weights (you can simply use the StAv keyword).

If you're only interested in the ground state and the lone pair orbital does not stay in the active state, perhaps you don't need it.

ABQTrap

Member

Registered: 2023-03-19

Posts: 24

Re: state-specific CASSCF opt structure rely on state number of SA(CIROOT)

The number of states affects the results, because the orbitals are optimized for a different average of states. It's a bit like the active space, there's no "right" answer, it depends on your system and what you're studying. In general, you should try to have all states well balanced, and I don't recommend changing the weights (you can simply use the StAv keyword).

If you're only interested in the ground state and the lone pair orbital does not stay in the active state, perhaps you don't need it.

Dear Ignacio, thank you for your prompt reply. I found the low-lying lone pair orbital of nitrogen atom which attaching to the methyl group may be essential for describing the N-N bond breaking process. So I tried to add the lone pair orbital into the acitve space. Correct me if I am wrong. Also, is there any rule of thumb for selecting the number of states of SA-CASSCF? Thank you in advance.

Last edited by ABQTrap (2024-10-18 02:05:21)