Subject: Using Stablized HF reference wavefunction or Unstalized?

From: "Zhenhua Li" <[email-masked]>
Date: Tue, 10 Apr 2001 11:07:41 +0800

Dear Listers,
I always meet difficulties of selecting reference wavefunction for
correlation methods such as MP2 when dealing with some singlet molecules.
For some singlet molecules, RHF wavefunction is unstable. Thus, it is
possible to get a stable UHF wavefunction by using the Stable function of
Gaussian98 etc. However, I often found that the MP2 or PMP2 energy based on
this stablized UHF reference wavefunction is higher than that based on
unstablized RHF reference wavefunction. So what reference wavefunction I
should use to deal with such molecules? Had we optimize the MP2 wavefunction
too (I am doubting if any program can do this.)?

Any comments?

Li Zhenhua

---

From: Xavier ASSFELD <[email-masked]>
Date: Tue, 10 Apr 2001 08:55:31 +0200

HI,

I often encounter that problem too.

The RHF wave function is generaly unstable because the system
has a strong multiconfigurational character. It results quite
often in a small HOMO-LUMO gap and then gives rise to near-degeneracy
problem. That is the reason why the RMP2 energy is so low.

The UHF solution is stable but has a very strong spin contamination
(<S^2> is close to 1). Then the UMP2 energy is higher than the
"exact" MP2 energy for the singlet. Moreover, du to self-interaction
term the HOMO-LUMO gap is large and thus the correlation correction
is small.

The PHF isn't better since the orbitals are not optimized. Then the
PMP2 energy presents "break points" on the potential energy
hypersurface.

I even wonder if the projected orbitals still satisfied the Brillouin 
theorem. I will check this last point.

What I did to deal with that problem is running "minimal" CASPT2
calculations on a small model system and check which MP2 (RMP2 or UMP2)
is the closest to the CASPT2 results. If the spin contamination is 
about constant for any geometry of the PES, then UMP2 gives quite 
accurate results.

Otherwise, you could use the EHF method of I. Mayer. (I.J.Q.C 7 (1973)
583)
I still do not understand why such a beautiful method is not implemented
in QM packages like Gaussian or Gamess or Jaguar or .... 

I would be very glad to read any other comments you will receive on 
this subject.

Hope this helps.
-- 

                                      ...Xav

WARNING! NEW E-MAIL ADDRESS !
(u-nancy changed to uhp-nancy)

Ast. Pr. Xavier Assfeld             [email-masked]
Laboratoire de Chimie theorique     (T) 33 3 83 91 21 49
Universite Henri Poincare           (F) 33 3 83 91 25 30
F-54506 Nancy BP 239                http://www.lctn.uhp-nancy.fr