Subject: REPORT ON A FREQ PROBLEM

From: "E. Lewars" <[email-masked]>
Date: Thu, 11 Jan 1996 22:30:31 -0500 (EST)

Hello, some interest was expressed in the problem (see below) I submitted to the
Net a couple weeks ago.
Some people suggested that the trouble lay in the symmetry changing during
Spartan's numerical freq calc (C4v --> Cs), or in instability of the wave-
function.  I tried to test the wavefunc stability, but that calc flopped with
a slew of "unable to assign spin for state..." messages.  Since G92 gave
the same, reasonable freqs with both analytical and numerical methods, I think
the "large imaginary" Spartan freqs are represent a (rare) quirk in that
program.  The pyramidane and several of its derivatives are relative minima,
with of sign of oddball freqs.
========================================

1995 Dec 25

Hello, I have an interesting problem connected with Spartan. I did an opt/freq
on the unusual molecule      Me            Me
                               \          /
  Me        /C\        Me        C------ C
   \      //  \ \     /          | \   /  |
     \ /-/------\-\ /            |   C    |    C(C-Me)4   Tetramethylpyramidane
      [/__________\]             | /   \  |
     /              \            C--------C    NOTE: pyramidane, C(CH)4, is
   /                  \        /            \        a relative minimum (no
 Me                    Me    Me              Me     imaginary freqs) at the
                                                    HF/6-31G*, MP2/6-31G* and
                        C4v                         MP2/6-31++G** levels.

           (The HF/6-31G* geom was given in the original mailing)

 The HF/6-31G* opt proceeded normally (E=-347.628 348), but the freqs are
 absurd:
        -68748.51   E
        -68748.51   E
        -22116.22   A1
        -11996.37   B1
        (followed by sensible +ve freqs
            151.29  B2
            151.88  A2
             etc.)

         ZPE = 566.02 kcal/mol

  Finding this hard to accept, I used the Spartan HF/6-31G* geom as input for
a Gaussian 92 HF/6-31G* opt/freq.  The Gaussian opt (which needed only one
opt cycle) gave the same E as Spartan (-347.628 348 6) but the freqs and ZPE
were quite different (the E from the freq job was also -347.628 348):

             +163.42  E
             +163.42  E
             +164.03  A2
             +164.64  B1
             +182.69  B2
                etc

            ZPE = 120.42 kcal/mol

  Has anyone any idea why the two programs gave such different results?
I presume that the "huge negative" freqs are simply wrong, and the Gaussian
freqs are correct; the parent compound C5H4, and other substituted pyramidanes,
gave normal freqs.

  Thanks for any suggestions.

 Errol Lewars