  We discuss a novel form of the variational approach in Quantum Field Theory
in which the trial quantum configuration is represented directly in terms of
relevant expectation values rather than, e.g., increasingly complicated
structure from Fock space. The quantum algebra imposes constraints on such
expectation values so that the variational problem is formulated here as an
optimization under constraints. As an example of application of such approach
we consider the study of ground state and critical properties in a variant of
nonlinear sigma model.
