Biquadratic interaction in calculation of classical spin energy

[mducle]

SpinW has a method energy which calculates the classical spin energy of a system, however the biquadratic interaction appears to have no effect on this. Cursory investigations shows that whilst the biquadratic terms are included in the sum they always sum to zero (withing round-off). We should investigate in the literature whether this is the correct behaviour.

Original reporter: L. Woodland