[molpro-user] Mixed derivatives in Numerical Hessian

[Leonid Shirkov]

Dear Colleagues,

in some specific cases, the current accuracy of the numerical hessians
is not enough, e.g. for very low frequency torsional vibrations (~50cm-1).
The CCSD(T) freq analysis gives imaginary values instead of the real ones
for the lowest modes.  The solution is to find manually the hessian in
the internal coordinates
and then find the eigenvalues of the GF matrix, but that is a lot of work.

If MP2 is used for such cases, then there are no imaginary frequencies.
Do I understand correctly, that for MP2 freq analysis the hessians are found
by differentiating the analytical MP2 gradients?

Using the analytical gradients for highly accurate methods like
CCSD(T) would probably resolve the problem,
but they are not currently available in Molpro.

Best regards,
Leonid

On Mon, Feb 6, 2017 at 10:18 AM, Werner Győrffy
<gyorffy at theochem.uni-stuttgart.de> wrote:
> Dear Aleksandr,
>
> Numerical Hessians in Molpro are computed by using central finite
> differences with a 2-point formula as a default. That is a "general
> formula". That gives accurate results in most of the cases. There is a
> trade-off between accuracy and efficiency: More accurate finite field
> calculations would increase the number of single point calculations
> significantly. If one needs more accurate Hessians, it can be done only by
> computing that manually by using procedures.
>
> Regards,
>
> Werner.
>
>
> On 02/04/2017 02:04 AM, Aleksandr Lykhin wrote:
>>
>> Does anybody know how Molpro calculates mixed derivatives using central
>> differences? It seems like it generates only two mixed displacements
>> instead of four, so the general formula cannot be applied directly.
>>
>> --
>> Kind regards, Aleksandr O. Lykhin.
>>
>>
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