This chapter outlines the theory underlying the calculations on which the remainder of this thesis is based.

*Ab initio* techniques, such as the method described herein,
attempt to solve the quantum mechanical equations governing the
behaviour of the system studied. In practice, the exact solution of
these equations is an intractable problem, hence a number of
approximations must be made. In contrast to semi-empirical approaches,
the approximations used in *ab initio* methods do not rely on
the specific nature of the system studied, but instead are based on
general physical principles. Thus, *ab initio* methods may be
transfered between widely differing applications with confidence (See
for example [2], [3] and [4]).

There are many complexities underlying the application of these
techniques to different classes of system. In this chapter we
will only discuss in detail those which are pertinent to the study of
biological and in particular, molecular systems. A full account may
be found in the article by Payne *et al.* [5].

Wed Sep 24 12:24:18 BST 1997