Introduction to variational methods

Abstract

The variational method is one of the most popular approaches to tackle quantum-mechanical few-body problems. Though it gives only an approximate solution except for some special cases (the Ritz variational method, for example, gives only an upper bound of the energy), one can get a virtually exact solution with an appropriately chosen function space. The function space is defined by basis states and the wave function of the system is expanded in that basis. In this chapter we briefly introduce the theorems requisite for obtaining a variational solution.