Molecular Mechanics

Abstract

For large molecular systems the methods of computational quantum chemistry described in Chapter 4 become intractable with present-day computing equipment and, indeed, with computers that may become available in the foreseeable future. By abandoning the strictly ab initio approach and developing semiempirical formalisms, we can extend the range of problems to which the methods of computational quantum chemistry can be applied. These semiempirical methods, such as the CNDO (Complete Neglect of Differential Overlap) method or MINDO (Modified Intermediate Neglect of Differential Overlap), usually involve some systematic neglect of two-electron integrals over the basis functions. These semiempirical methods are faster than the corresponding ab initio calculations by perhaps one or two orders of magnitude for small molecules and by more for larger systems. However, for large molecules with many degrees of freedom even these semiempirical approaches become intractable. We are still a long way from the ideal of obtaining chemical results by solving the Schrödinger equation for a system of arbitrary size and complexity.