Molcas Forum

mradon

Member

Registered: 2022-02-17

Posts: 3

MP2 based on ROHF wave function

Dear All:

Is it possible to perform MP2 calculations based on high-spin ROHF reference function? The equivalent of RMP2 in Molpro, ROMP2 in Gaussian.

The MBPT2 program is limited to closed-shell RHF references. I thought about using the CASPT2 program based on the ROHF calculations performed with the RASSCF program (only 1 configuration in CASSCF, obtained by making active only singly occupied orbitals). However, the CASPT2 energies differ from the ROHF-MP2 ones by many kcal's/mol. I guess it has to do with the form of the 0th-order Hamiltonian currently used in Molcas (the IPEA stuff).

Is it possible to perform CASPT2 calculations in such a way that they are equivalent to ROHF-MP2 in this special case where CASSCF reduces to ROHF?
Or is there any other way to obtain correct ROHF-MP2 energies in Molcas?

Thank you in advance!

Regards,
Mariusz Radoń

mradon

Member

Registered: 2022-02-17

Posts: 3

Re: MP2 based on ROHF wave function

Anyone?

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,085

Re: MP2 based on ROHF wave function

You can always set IPEA=0.0 if that's the problem.

mradon

Member

Registered: 2022-02-17

Posts: 3

Re: MP2 based on ROHF wave function

Dear Ignacio:
Thank you for your reply. Yes, I can do CASPT2 with IPEA=0.0, but the energy is different from the expected one (from other programs)!

For instance, O2 (distance 1.208 Angstrom), cc-pVDZ basis set, triplet ground state (total energies in a.u.):

• Molpro: ROHF=-149.60798659, MP2=-149.98700133 # hf rmp2

• Gaussian: ROHF=-149.60798659, MP2=-149.98701565 # ROMP2(FreezeNobleGasCore)

• OpenMolcas: CASSCF(2,2)=-149.60798384, CASPT2(IPEA=0)=-150.01567487, CASPT2(default IPEA) = -149.99690639 # OpenMolcas v21.06

As you can see, although all the ROHF / CASSCF(2,2) energies agree and the ROHF-MP2 energies agree between two other programs, the CASPT2(IPEA=0) energy is not the same. The same core orbitals (two O 1s) are frozen in all cases. Analogous comparison for closed-shell N2 gives identical MP2 and CASPT2 energies (identical to within 4e-6, this small difference is probably due to the Cholesky decomposition used in Molcas, but not in other programs).

So, how can I make CASPT2 equivalent to MP2 for open-shell systems?

Best regards,
Mariusz Radoń