Is second‐order perturbation theory sufficient to treat second‐order properties?

If low-order perturbation theory is used to approximate physical quantities, the linear series has in general limited validity. In particular, the second order contribution, when examined in the light of variation-perturbation theory, has the form ϵe2γ, with γ usually well approximated by (1 - ϵe3/ϵe2)−1. Applications of this technique to dispersion forces, polarizabilities, and susceptibilities is rewarding.