Hartree-Fock theory is the oldest and most well-known ab initio electronic structure method. For a comprehensive presentation of HF theory, and its applications to the prediction of molecular properties, the reader is referred to the 1986 monograph [1] and references within. HF theory has the advantage that it is the simplest ab initio theory, and therefore is the least computationally expensive. In many cases, however, the accuracy of the HF method is not sufcient due to its incomplete description of the correlation between motions of electrons, and account must be taken of these effects in order to improve the quality of the results. More sophisticated ab initio methods, which include electron correlation, such as perturbation theory, conguration interaction, and coupled-cluster techniques, tend to be expensive in terms of computational resources and, at present, are not practical for large systems.