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.

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?

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.

James Kress Ph.D., President
The KressWorksR Institute
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