  We study the quantum phase transition in optical lattices using ordinary Bose
Hubbard Hamiltonian within two loop approximation in variational perturbation
theory. We have shown that this approximation can reproduce superfluid Mott
insulator transition in contrast to the simple Bogoliubov or Hartee - Fock -
Popov approximations.
  The superfluid fraction and ground state energy per particle vs input
parameters of the model are studied.
