Effective potential for quantum correlation functions

A method for calculating quantum averages and static correlation functions of configurational observables is introduced, by extending a variational method, which permits the reduction of the problem to the computation of classical-like averages. This approach opens the possibility of obtaining novel results in many fields, and strongly shortens the computer time needed for numerical calculations. As an example, the well-known sine-Gordon model is considered, and its relevant quantum correlation functions are calculated on the whole temperature range.