Molcas Forum

Okko

Member

Registered: 2023-04-27

Posts: 8

Geometry optimization for an UMP2 calculation

Dear all,

I want to perform a geometry optimization at an unrestricted MP2 (UMP2) level of theory. For a test case I tried to optimize the oxygen molecule using a triplet state:

 &GATEWAY
  Title= O2
  Coord= $CurrDir/oxygen.xyz
  Basis set
  ANO-RCC-VTZP

>> DoWhile

&SEWARD
cholesky

&SCF
UHF
Spin = 3
End of Input

&MBPT2
End of Input

&SLAPAF
MAXSTEP
0.01
CARTESIAN
End of input

>> EndDo

However, I realized that the MBPT2 module gives a waning message:

 ###############################################################################
 ###############################################################################
 ###                                                                         ###
 ###                                                                         ###
 ###    WARNING: MP2 implementation intended for RHF references only         ###
 ###                                                                         ###
 ###                                                                         ###
 ###############################################################################
 ###############################################################################
MBPT2 WARNING: Reference function according to RunFile:UHF-SCF 
I'll assume you know what you're doing and continue

The calculation terminates normally but I am a bit worried about the warning and that the reference wave function for the MP2 calculation is not based on the "UHF" but instead contiunes on with the "RHF" wave function.

In other words, can I ignore this warning or do I need to switch to the CASPT2 method?

Thanks in advance!