Re: CCL:Problem with O3 freq.
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:
>
> 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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