Subject: Different UMP2 results with G90 and G92.

From: <[email-masked]>
Date: Sat, 4 Dec 1993 13:30:35 -0600 (CST)

Hi, CCL netters,

I have found a rather puzzling and disturbing difference between results 
obtained by Gaussian90 and Gaussian92 for geometry optimisations at
UMP2 level. I have been looking at FO2. I tried to duplicate the 6-31G*
results of Frank Jensen, Chem Phys Lett 169, 519, 1990.

G92 gives his UMP2 result, OF = 1.3832; OO = 1.2497 amd FOO angle = 109.6.
This is with UMP2=FULL. G90 however, with UMP2 (FULL is default) gives
results very close to the ones described by Jensen as AUMP2 (which were 
frozen core) - OF = 1.6894 (1.6985); OO = 1.1669 (1.1641); FOO angle =
113.3 (113.5) - Jensen values in (). AUMP2 appears to do the MP2 expansion 
on the UHF wave function after spin projection, while UMP2 does it before 
spin projection (i.e. on the straight UHF wave function). The frequencies
likewise essentially agree with Jensen's UMP2 and AUMP2 results, as do the
average values of S^2 before and after projection and the total energies
(although the G90 UMP2 FULL is lower as expected than Jensen's AUMP2 frozen 
core). The two calcualtions both started with OPT=CALCFC from a value near
to the UMP2 G92 result. Starting from the G90 geometry, G92 has FOO falling
apart, with the F dissociating from O2 exactly as happens at UHF level.
This is how I found this by trying to do the frequency with G92 from the
G90 geometry on a differennt machine - it gave nonsense so I reoptimised.

I can find no mention in the gaussian literature about this difference.
Can anyone throw any light on it? It does seem that G90 is doing what
Jensen calls AUMP2, while G92 is doing straight UMP2. I would most
welcome any comments on this.

Regards to everyone. Brian.
-- 
        Associate Professor Brian Salter-Duke (Brian Duke)
School of Chemistry and Earth Sciences, Northern Territory University,
Box 40146, Casuarina, NT 0811, Australia.  Phone 089-466702
e-mail: [email-masked]  or [email-masked]

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From: [email-masked] (Michael Frisch)
Date: Sat, 4 Dec 93 08:39:31 EST

Brian Salter-Duke asks:
    Hi, CCL netters,

    
    
Both G92 and G90 do regular UMP2, period.  The only diffence between
them other than peformance is that G92 can do frozen-core MP2 gradients
and, as Brian notes, therefore does frozen-core optimizations by
default.  The projected (PMP) energies printed by Gaussian are not used
during geometry optimization.

Note, however, that CALCFC means different things in G90 and G92.
G90 could only do SCF analytic second derivatives, and CALCFC meant
"do the SCF Hessian at the beginning of the optimization" regardless
of whether HF, MP2, QCISD, or whatever was being optimized.  Since
G92 can the MP2 Hessian as well, there are now two versions of this
options:  CALCFC means what it says:  calculate the force constants
at the level of theory for the optimization (i.e., HF for an HF
optimization, MP2 for an MP2 optimization).  CALCHFFC means do the
SCF force constants regardless of the level used for the optimization.
I would speculate that the two optimizations took different paths
because of different initial Hessians.  If you started with the
bond lengths on the long side of equilibrium, then whether the
optimization moved inward or dissociated could change with whether
the optimizer thought it was in a region of positive or negative
curvature, which might well be different for the SCF and MP2 Hessians.

Mike Frisch
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