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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