I am looking for a complete description of the interaction of two traceless cartesian quadrupoles. Even just an implementation online would be quite useful. The problem I am having is many of the implementations I am aware of are quite opaque given that they are intended for use in MD and hence are quite optimized and often include terms related to ewald summation.
I am also aware of the appendix in Anthony Stone's book which gives the interaction tensors in terms of components of direction cosines. Is there any more compact form for the evaluation of the interaction of two quadrupoles than the somewhat daunting equations given there?
I am pretty surprised I can't find a straightforward implementation of this for a pair of traceless quadrupoles in cartesian coordinates. Any reference would be appreciated.
If there just isn't an easier way than using the expressions provided by Stone, then I suppose that's the way I'll have to go.