  A method for nonperturbative path integral calculation is proposed.
  Quantum mechanics as a simplest example of a quantum field theory is
considered. All modes are decomposed into hard (with frequencies $\omega^2
>\omega^2_0$) and soft (with frequencies $\omega^2 <\omega^2_0$) ones,
$\omega_0$ is a some parameter. Hard modes contribution is considered by weak
coupling expansion. A low energy effective Lagrangian for soft modes is used.
In the case of soft modes we apply a strong coupling expansion. To realize this
expansion a special basis in functional space of trajectories is considered. A
good convergency of proposed procedure in the case of potential $V(x)=\lambda
x^4$ is demonstrated. Ground state energy of the unharmonic oscillator is
calculated.
