Factorization of static perturbation theory for weakly coupled systems using imaginary time

Starting from the interaction picture in quantum mechanics it is shown that perturbation theory for a weak static coupling of two subsystems in a natural way can be reformulated through integrals over products of imaginary time cross correlation functions. This leads to an expression for the London dispersion energy contribution of intermolecular interactions offering new ways to its numerical computation. The frequency-dependent charge susceptibility functions at imaginary frequencies appearing in the Casimir-Polder expression for the dispersion energy and the imaginary time density fluctuation cross correlation functions occuring in the equivalent alternative formula turn out to be Fourier cosine transform pairs.