Molcas Forum

SANA123456

Member

Registered: 2025-06-01

Posts: 2

ROHF vs UHF

Dear All,

I am a new user of OpenMolcas  and have been conducting calculations on the FeH molecule, incorporating the Douglas-Kroll-Hess (DKH) relativistic correction.
I observed that the energy from the Unrestricted Hartree-Fock (UHF) calculation (Total SCF energy: -1271.4458522848 au) aligns well with results from other computational packages. However, the Restricted Open-Shell Hartree-Fock (ROHF) calculation yields significantly different results:

Average CI energy: -1204.89488278 au
RASSCF energy for state 1: -1204.89488278 au

These discrepancies are unexpected. I have attached the input files for both calculations for your reference.
Could you please advise on the possible reasons for this discrepancy? Specifically, are there any options or settings I should adjust in the ROHF calculation to achieve energy results comparable to those from the UHF calculation?

Thank you for your assistance.

----------------------------------------------------
ROHF input file
----------------------------------------------------

&GATEWAY
  Title = FeH Ground State UHF Calculation
  Symmetry
    XY  Y

 Douglas-Kroll

 Basis set
    Fe.CC-PVTZ-DK...
    Fe 0.00000 0.00000 0.000000 bohr
  End of Basis

  Basis set
    H.CC-PVTZ-DK...
    H 0.000000 0.000000 1.600000 bohr
  End of Basis

End of Input

&SEWARD
End of Input

&RASSCF
  Title = FeH Ground State ROHF Calculation
  Spin = 4
  Nactel = 3 0 0
  Inactive = 7 0 3 2
  Ras2 = 1 0 1 1
  Ciroot = 1 1 1
End of Input

----------------------------------------------------
UHF input file
---------------------------------------------------

&GATEWAY
  Title = FeH Ground State UHF Calculation
  Symmetry
    XY  Y
 Douglas-Kroll

 Basis set
    Fe.CC-PVTZ-DK...
    Fe 0.00000 0.00000 0.000000 bohr
  End of Basis

  Basis set
    H.CC-PVTZ-DK...
    H 0.000000 0.000000 1.600000 bohr
  End of Basis

End of Input

&SEWARD
End of Input

&SCF
  Title = FeH Ground State UHF Calculation
  UHF
  Charge = 0
  zSpin = 3
End of Input

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,204

Re: ROHF vs UHF

Is the symmetry of your RASSCF state correct? If I understand it correctly, you want singly occupied orbitals of symmetries 1, 3 and 4. Shouldn't the state symmetry be 2 in that case?

SANA123456

Member

Registered: 2025-06-01

Posts: 2

Re: ROHF vs UHF

Yes, you are right.
I checked the symmetries and there was a mistake.
It works now with the following input file.

&GATEWAY
  Title = FeH Ground State UHF Calculation
  Symmetry
    XY  Y

 Douglas-Kroll

 Basis set
    Fe.CC-PVTZ-DK...
    Fe 0.00000 0.00000 0.000000 bohr
  End of Basis

  Basis set
    H.CC-PVTZ-DK...
    H 0.000000 0.000000 1.600000 bohr
  End of Basis


End of Input

&SEWARD
End of Input


&RASSCF
  Title = FeH Ground State ROHF Calculation
  Spin = 4
  Nactel = 3 0 0
  Inactive = 7 3 0 2
  Ras2 = 1 0 1 1
  Ciroot = 1 1 1
End of Input

Thanks

Last edited by SANA123456 (2025-06-02 17:10:38)