Semiempirical Calculations

Abstract

Semiempirical quantum mechanical calculations are based on the Schr€ odinger equation. This chapter deals with SCF semiempirical methods, in which repeated diagonalization of the Fock matrix refines the wavefunction and molecular energy. These calculations are much faster than ab initio ones, mainly because the number of integrals to be dealt with is greatly reduced by ignoring some and approximating others with the help of experimental quantities, or values from high-level ab initio or DFT calculations. In order of increasing sophistication, these SCF semiempirical procedures have been developed: PPP (Pariser-Parr-Pople), CNDO (complete neglect of differential overlap), INDO (intermediate neglect of differential overlap), and NDDO (neglect of diatomic differential overlap). Today the most popular SCF semiempirical methods are AM1 and PM3, which are carefully parameterized to reproduce experimental quantities (primarily heats of formation). Recent extensions of AM1 (RM1) and PM3 (PM6) seem to represent substantial improvements and are likely to soon become the standard semiempirical methods. 6.1