For simplicity consider the closed chain Ising model - a string of n neighbor
coupled spin sites 
joined at the ends. Thus spin
is coupled to spin 
. The spin values 
take on the values -1 or 1. The energy of the system is given by the sum of the internal interaction energies of neighboring spin states and the energy of interaction of the set of states with an external field, which for the Ising model is taken to be a sum of spin-field energies. Thus, with the spin-spin coupling given by -Q and the spin-field constant -R, the energy in the Ising model is given by
