[gmx-users] second derivative of force
Nguyen Hoang Phuong
phuong at theochem.uni-frankfurt.de
Wed Jun 21 09:01:25 CEST 2006
Hi,
thanks Mark for the suggestion. I made an mistake in writing the first
email. Actually, I need the first derivative of force, i.e,
the second derivative of total energy.
Perhaps, a modification the code that computes the Hessian is a good idea.
To do this, does anyone know exactly where is the code such that I just
add the if command
if(i==j){
...calculating the Hessian
}
Many thanks for your help.
Phuong
>
> I don't think you'll find any short-cut, because I can't see that there is
> another use for this type of calculation, hence nobody will have included it
> already in GROMACS. Hessian matrices are expensive to calculate for QM
> because they scale with something like the fourth power of the number of
> basis vectors. The cost of finding Hessian matrices in MM is roughly linear
> in the number of atoms, so it ought not to be too scary. In fact, it would
> only be more expensive by a factor of N over an algorithm that only
> calculates the diagonal terms. If it is too expensive, you also might want to
> reconsider doing it at every trajectory snapshot :-) Otherwise, your only
> recourse will be to find the piece of code that computes the Hessian, and
> rework the loops over i and j so that i=j always.
>
> Mark
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