Functional minimization method addressed to the vacuum finding for an   arbitrary driven quantum oscillator
  The old problem exists for a driven (time-dependent) quantum oscillator: to differ the true vacuum state from the squeezed one. We suggest finding the true vacuum state by minimization of the functional containing the difference of the potential and kinetic energies of oscillator. Analytical and numerical examples confirming this offer are considered. 
S.V. Anischenko, S.L. Cherkas, V.L. Kalashnikov