Introduction

We have developed techniques by which the general energy eigenvalue problem can be reduced to a set of coupled partial differential equations involving various wave-functions. Unfortunately, the number of such problems which yield exactly soluble equations is comparatively small. Clearly, we need to develop some techniques for finding approximate solutions to otherwise intractable problems.

Consider the following problem, which is very common. The Hamiltonian of a system is written

$$H = H_0 + H_1.$$ (587)

Here, $H_0$ is a simple Hamiltonian for which we know the exact eigenvalues and eigenstates. $H_1$ introduces some interesting additional physics into the problem, but it is sufficiently complicated that when we add it to $H_0$ we can no longer find the exact energy eigenvalues and eigenstates. However, $H_1$ can, in some sense (which we shall specify more exactly later on), be regarded as being small compared to $H_0$. Can we find the approximate eigenvalues and eigenstates of the modified Hamiltonian, $H_0+H_1$, by performing some sort of perturbation analysis about the eigenvalues and eigenstates of the original Hamiltonian, $H_0$? Let us investigate.