[molpro-user] Mixed derivatives in Numerical Hessian

[Benj FitzPatrick]

Werner,
Addressing the edge of curiosity, will open-shell CCSD/CCSD(T) also be
getting analytic gradients?
Thanks,
Benj FitzPatrick

On Tue, Feb 7, 2017 at 5:24 PM, Werner Győrffy <
gyorffy at theochem.uni-stuttgart.de> wrote:

> Dear Aleksandr,
>
> I agree that the thresholds must be made tighter: especially for
> Hartree-Fock, and also for CP-HF in the case of using analytical gradients.
> One should be careful not to set the thresholds too tight. For example, the
> suggested "gthresh,orbital=1.0d-10" is equivalent to "{hf;accu,20;}" which
> seems to be unnecessary low and might cause convergence problems.
>
> Yes, MP2 and DF-MP2 Hessians are computed by using analytical first
> derivatives.
>
> CCSD(T) analytical gradients have been already implemented in the
> development version of Molpro. It will be hopefully available soon in the
> next release.
>
> Regards,
>
> Werner.
>
> On 02/06/2017 07:14 PM, sjk wrote:
>
>> 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
>> <mailto: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
>>> <mailto: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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>>
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