Perturbation Method beyond the Variational Gaussian Approximation: The   Liouville-Neumann Approach
  We have developed a variational perturbation theory based on the Liouville-Neumann equation, which enables one to systematically compute the perturbative correction terms to the variationally determined wave functions of the time-dependent systems. We then apply the method to the time-independent anharmonic oscillator, and show that the results agree with those of other variational perturbation theories. We also show that the system has an interesting algebraic structure at the first order correction level. 
Dongsu Bak, Sang Pyo Kim, Sung Ku Kim, Kwang-Sup Soh, and Jae Hyung   Yee