Rayleigh-Schr\"{o}dinger Perturbation Theory Based on Gaussian Wavefunctional Approch

A Rayleigh-Schr"{o}dinger perturbation theory based on the Gaussian wavefunctional is constructed. The method can be used for calculating the energies of both the vacuum and the excited states. A model calculation is carried out for the vacuum state of the $\lambda {\varphi }^4$ field theory.