  The effective action for the local composite operator $\Phi^2(x)$ in the
scalar quantum field theory with $\lambda\Phi^4$ interaction is obtained in the
expansion in two-particle-point-irreducible (2PPI) diagrams up to five-loops.
The effective potential and 2-point Green's functions for elementary and
composite fields are derived. The ground state energy as well as one- and
two-particle excitations are calculated for space-time dimension $n=1$, when
the theory is equivalent to the quantum mechanics of an anharmonic oscillator.
The agreement with the exact spectrum of the oscillator is much better than
that obtained within the perturbation theory.
