It is one of the effective research method to obtain realistic classical interatomic potential or force field for molecular dynamics simulation with high degree of transferability for systems which the first principles or ab initio method is capable of treating.
This method is based on fitting the forces on individual atoms in a number of reference structures, cohesive energies and stresses on unit cell obtained from first principles calculation with those obtained from classical interatomic potential.
The target of the computational fitting is to determine unknown coefficients in classical interatomic potential function.
Differentiating the L-J potential with respect to r gives an expression for the net inter-molecular force between 2 molecules.
This inter-molecular force may be attractive or repulsive, depending on the value of r. When r is very small, the molecules repel each other.