Briefly, this code calculates coupling constants using the following Hamiltonian: [Heisenberg], [Dirac], [Vanvleck]

    Ĥ = -2 ∑_i>j J_ijs_is_j

where the terms have the following meanings:

Jij		The exchange coupling constant
sx		The spin operator.

Any definition of spin operators (formal spins or spin projections or spin densities or any other) can be used. Sometimes researchers exclude the pre-factor 2 in the above formula so this needs to be kept in mind while using this code.