Subject: Problem with O3 freq.

From: Tapas Kar <[email-masked]>
Date: Mon, 10 Aug 1998 12:26:26 -0500 (CDT)

Hi everybody,

Here is the expt vib. frequencies of O3
  705  1110  and  1042  ( Ref. Herzberg )
Theoretical values are (output of mp2(full)/6-31+G*)
                    A1                     A1                     B2
 Frequencies --   725.8632              1162.4426              2391.7224
 Red. masses --    15.9949                15.9949                15.9949
 Frc consts  --     4.9653                12.7343                53.9081
 IR Inten    --     6.5303                 2.4236              1635.3846

In fact we tried with other basis sets like 4-31G, 6-311G* at MP2 level
and
the values are close to the above values.
Clearly 2391 (B2 mode) is too high. what is the problem here.
Tapas
~

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Tapas Kar, Ph. D                           
Asst. Scientist/Asst. Professor (Adjunct)                            
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From: Anatoli Korkin <[email-masked]>
Date: Mon, 10 Aug 1998 19:09:17 -0700

Tapas,

The problem is that for highly correlated systems like O3, for example,
MP2 (or MBPT[2]) is not "the word from Bible". You need to apply coupled
cluster (see Watts & Stanton  & Bartlett, CPL, 1991, 178, p. 471) or
multi-reference methods to get correct results. To find out when you
may get "a trouble" look at LUMO energy, and if it is negative the
system
is highly correlated. You may also see that your IR intensity is 
unrealistically high, which shows that perturbation theory is not
working "properly".

Chemical systems, which are "under suspition" for
high correlation effects are:

1) electron rich systems;
2) transition states;
3) system with "unusual" coordination numbers;
4) unique Lewis structure can not be drawn;
5) condensed multiple bonds;
6) radicals and biradicals.

Of course, many systems fall into a few categories.

Anatoli

P.S. B3LYP is not so bad after all in this case even it is about
or even faster than MP2. At least you get them in a right order
(B3LYP/6-31+G*):

                   A1                     B2                     A1
 Frequencies --   733.1291              1220.1424              1257.5397
 Red. masses --    15.9949                15.9949                15.9949
 Frc consts  --     5.0652                14.0299                14.9031
 IR Inten    --     6.3178               232.6091                  .0020
 Raman Activ --      .0000                  .0000                  .0000
 Depolar     --      .0000                  .0000                  .0000

With CCSD (which is more expensive) you get 748, 1240 and 1256 (Stanton
et al, JCP, 1989, 90, p. 1077.

Regards,

Anatoli

Tapas Kar wrote:
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