Smooth Perturbations to R\'enyi Entropy

A method is presented for computing the R\'enyi entropy of a perturbed massless vacuum on the ball via a comparison with lattice field theory. If the perturbed state is Gaussian with smoothly varying correlation functions and the perturbation parameter has units of energy, I show the coefficients for R\'enyi entropy are analytically computable for all R\'enyi parameter $\alpha$ in odd dimensions and for integer $\alpha$ in even dimensions. I apply this procedure to compute coefficients for the large distant expansion for the R\'enyi mutual information of distant balls and the low temperature expansion for the entropy of a thermal field.