ABSTRACT

To this point we have focused our attention on single-determinant wave functions. Such wave functions are of course approximate, but for the conventionally bound molecules we have studied, the single determinant is a reasonable starting point. There are, however, important chemical contexts when the single determinant is inadequate for our purposes. Whenever the correlation error of the HF method is quantitatively significant, or when the single determinant is not even qualitatively suited to a particular system, we cannot be contentwith the single-determinant approximation. It is possible to write a more flexible and competent wave function by mixing severalperhaps many-determinants. This is called configuration interaction.