Application of the FAKE molecular-orbital method to diatomic molecules XY (X, Y = H, F, Cl, Br, I)

The FAKE (fast, accurate kinetic energy) method of semiempirical molecular orbital calculation is applied to diatomic molecules XY (X, Y= H, F, Cl, Br, I). The method differs from the extended Hückel theories in that it applies simple approximations only to the potential energy integrals, while including accurately calculated kinetic energy effects. This more appropriate treatment of the kinetic energy renders unnecessary the adjustment factors ordinarily introduced in Wolfsberg-Helmholz and Cusachs approximations to obtain a reasonable description of chemical bonding and leads to iterative procedures with greatly improved convergence characteristics. From our present application of FAKE calculations to diatomic molecules we found that the method mimics "real" selfconsistent-field studies to a considerable accuracy; however, the method is primarily designed for our work on large molecules and polymers, for which only the most rapid of semiempirical methods are practical.