ABSTRACT
There are very few problems which have exact solutions in quantum mechanics. How will we proceed to describe a molecule when even the He atom does not have an exact solution? Two powerful techniques, the variation method and the perturbation method, are in wide use. The variation method is used most widely to characterize molecular energies while perturbation theory is applied to molecular properties. Almost every calculation in quantum chemistry begins with an application of the variation principle-even estimates of molecular properties, defined as a system’s response to some disturbance, requires as a starting point some description of the undisturbed system. This description will always be founded on a variational method. Here we explore the variational theorem and a variety of illustrations, leaving the perturbation methods for later attention.
