A novel variational approach for Quantum Field Theory: example of study   of the ground state and phase transition in Nonlinear Sigma Model
  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. 
Yuriy Mishchenko, Chueng-Ryong Ji