Dear Amir,

First of all, you have to determine some basic properties of your material such as lattice constant, lattice energy per atom, elastic modulus and so on, base on first principle calculations or real data (reference properties).

Then make your model in GULP and set optimization simulation such as the properties of your material and your reference properties difference should be goes to zero. Gulp try to find your potential function parameters base on reference properties.

More reference properties more accuracy.

There are some available codes able to do the parameterization for several classical force fields. Gulp is one of them. You may search on internet for others: atomicrex, dftfit, GAFit, GARFfield, MEAM-fit. You have to be familiar with the force field itself and quantum chemistry and/or solid-state calculation methods that are necessary for calculation of the reference data.

You can do an automatic parameterization with GULP and DMol3 inside Materials Studio.

See also the FF optimizer in MedeA

http://www.materialsdesign.com/medea/forcefield