Subject: G:Wavefunction Instability

From: "M. Nicklaus" <[email-masked]>
Date: Tue, 1 Oct 1996 11:50:30 -0400

Dear CCL'ers,

I am calculating, at the HF/6-311G** level in Gaussian 94, the divalent cation
Mg++ complexed with the hydroxyl groups of catechol.  Both catechol and the    
ion (should) have a spin multiplicity of 1, so that's what I used for the complex.
   At the current geometry, which was optimized with a smaller basis set, I 
performed a wavefunction stability test (keyword "Stable").  I got the message
          The wavefunction has an RHF -> UHF instability.
Running a "Stable=Opt" job produced a stable wavefunction at a slightly lower
energy.  The differences are not huge: the energy is lowered by 1.45 kcal/mol,
and the changes in the charge on the Mg++ ion (both Mulliken and NPA) are less
than 10^-3.  (Interestingly, the complex of Mg++ with the aromatic ring of  
the catechol molecule did not show any wavefunction instability.)  Using UHF
instead of (R)HF, while keeping mult. = 1, produced exactly identical results
compared with the HF calculations, both with and without the keyword "Stable=Opt".
The spin contamination produced by the wavefunction optimization is
 Annihilation of the first spin contaminant:
 S**2 before annihilation     0.3882,   after     0.2298

Before I burn weeks' worth of CPU cycles on potentially unnecessary and/or
meaningless calculations, I'd like to ask for advice on:

(a) Are these results a reason for concern, i.e. does S**2 = 0.3882/0.2298 mean
    that the optimized wavefunction is useless; or, on the other hand, does the 
    small energy change of only 1.45 kcal/mol mean that I shouldn't even bother
    optimizing the wavefunction?

(b) Since I intend to optimize the geometry of the complex at the HF/6-311G**
    level, is it possible the the wavefunction instability will go away anyway 
    after that?  [I guess I could simply try that, but see (c).]

(c) If I (should) use "Stable=Opt", how do I combine that with the geometry
    optimization?  Can you use both in the same job?  If so, when does G94 do
    the wavefunction optimization; i.e., (only) for the initial geometry, 
    and/or for each intermediate geometry, and/or (only) for the final geometry?
    This would appear to have a major impact on the CPU time used.  If you can't
    combine "Stable=Opt" and geometry optimization in a single job, how should 
    one proceed?

(d) Any other comments, ideas, suggestions or warnings concerning this problem?

I'll summarize useful answers.  Thanks in advance to all who'll respond.

Marc

------------------------------------------------------------------------
 Marc C. Nicklaus                        Lab. of Medicinal Chemistry
 e-mail: [email-masked]               National Cancer Institute, NIH
 Phone:  (301) 402-3111                  Bldg 37, Rm 5B29
 Fax:    (301) 496-5839                  BETHESDA, MD 20892-4255    USA
         WWW:  http://www.nci.nih.gov/intra/lmch/MCNBIO.HTM
------------------------------------------------------------------------