# Force constants

Please respond to the list.

I still don’t know what you are asking about. (coefficients of some cubic polynomial that you’ll use as a pair style/force law?) My response was just trying to get you to you reformulate your question, so that you would get answers to your question from someone knowledgeable. It’s impossible for anyone to know what you mean (without a lot of guess work) with such little info. In any case, with this amount of information, this doesn’t really seem to be a lammps question - unless you can convince otherwise.

Dear Eric,

Let me be clear.
Is there any command such as fix or compute in Lammps to calculate the energy using , extract coordinate and energy information from lammps and then take second derivatives with respect to displacement to calculate force matrix using finite difference approach?

Best regards,

Nima Pirouzmand

If you mean a full Hessian of 2nd derivatives, then no

such compute/fix yet exists. You can look at the

USER-PHONON package which computes a dynamical

matrix from a force matrix which may be a related

kind of computation.

Steve

Dear Eric,
Let me be clear.
Is there any command such as fix or compute in Lammps to calculate the energy using , extract coordinate and energy information from lammps and then take second derivatives with respect to displacement to calculate force matrix using finite difference approach?

do you see any command in the documentation that looks like it could do that??

at any rate, what you are asking for is very obviously straightforward
to realize using the library interface, e.g. using python scripting.
if you search through the mailing list archives, you'll see that there
have been people asking questions related to doing something along
those lines and i think that there is a good chance that one of the
higher level atomic simulation toolkits (e.g. ASE) have a module for
such things.

axel.

Thank you Steve for your comment.

Best regards,

Nima Pirouzmand

You may be able to part of the Tinker suite to use the data from LAMMPS to
compute the force constants.

http://dasher.wustl.edu/tinker/

Jim

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Dear James,

Thank you very much for introducing Tinker.

Best regards,

Nima Pirouzmand