A TEST OF THE NEW INTEGRATED MO-BUTANE(MM (IMOMM) METHOD FOR THE CONFORMATIONAL ENERGY OF ETHANE AND N)

Test calculations of the newly developed ''Integrated Molecular Orbita l + Molecular Mechanics'' (IMOMM) method were performed for the optimi zed equilibrium and transition structures and energies of ethane and n -butane. In this method, the total energy of a large molecular system is expressed as a sum of the MO energy of the small ''model'' system a nd a modified MM energy of the ''real'' system, and full geometry opti mization is carried out using the gradient of this total energy. Vario us schemes of partition of the system into the MO part and the MM part , including some not intended in the original design of the method, we re examined and compared with the pure ab initio MO and the pure MM re sults. In most reasonable partition schemes, the IMOMM method can repr oduce the pure ab initio and the pure MM geometries and energies quite well. (C) 1996 John Wiley & Sons, Inc.