Dear friends,
       Can any body guide me how to make the program 
in FORTRAN77/90 or in C on square lattice for the 

following quantum spin (anti-ferromagnetic) model:  

    H = J*nnsum(i,j)*S_{i}*S_{j} + 

        J'*sum(i)*sum(delx)*sum(dely)*[(S_{i}*S_{i+delx})*(S_{i+delx+dely}*S_{i+2*delx+dely})]

First term is simply Heisenberg term and second is
 another
type coupling term.
Thanking you,