Hi Dac,
Many thanks for your input, I appreciate it.
Completely agree with you, which is why I am extremely confused how a
number (and not a small one) of publications have claimed to use second
order derivation of cartesian coordinates (hessian) from QM calculations.
An extensive example: "Systematic Derivation of AMBER Force Field
Parameters
Applicable to Zinc-Containing Systems². In their abstract:
³...In this study, we have derived bond stretching and angle-bending
parameters applicable to zinc-containing systems which are compatible with
the AMBER force field. A total of 18 model systems were used to mimic the
common coordination configurations observed in the complexes formed by
zinc-containing metalloproteins. The Hessian matrix of each model system
computed at the B3LYP/6-311++G(2d,2p) level was then analyzed by
Seminario¹s method to derive the desired force Constants.."
Many thanks
Anthony
On 15/10/2015 17:35, "David A Case" <dacase.scarletmail.rutgers.edu> wrote:
>On Thu, Oct 15, 2015, Nash, Anthony wrote:
>>
>> the force constant for A-B is not the same as B-A.
>
>> So using second
>> order tensors, the force constant for C-H is much greater than the
>> force constant for H-C (where C is seen is the displacing atom, and H
>> experiences the force).
>>
>
>This is not the way molecular mechanics potentials work. The bond energy
>doesn't know anything about what is the "displacing atom". It just looks
>at the distance from one atom to the next (which clearly doesn't depend
>upon
>which order is used.)
>
>If you wish to fit molecular mechanics potentials to quantum data, there
>are
>lots of tools that will help do that. But I don't know of any that start
>from
>the sort of non-symmetric tensors that you are thinking of.
>
>Maybe someone on the list will be able to chime in.
>
>....dac
>
>
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Received on Thu Oct 15 2015 - 12:00:05 PDT
