[molpro-user] Mixed derivatives in Numerical Hessian

[sjk]

I find that tightening the convergence criterion on various evaluations generally solves the low frequency hessian convergence problems (my experience is that anything below about 200 cm-1 should not be considered reliable with the standard parameters).
Thus, I generally insert the following line at the top of my input file
gthresh,energy=1.0d-10, orbital=1.0d-10, oneint=1.0d-16, twoint=1.0d-16, optgrad=1.0d-6, compress=1.0d-13
Best Regards,
	Stephen
On Feb 6, 2017, at 9:12 AM, Leonid Shirkov <leonid.shirkov at gmail.com> wrote:

> 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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