A test of the new ?integrated MO + MM? (IMOMM) method for the conformational energy of ethane andn-butane

Test calculations of the newly developed “Integrated Molecular Orbital + Molecular Mechanics” (IMOMM) method were performed for the optimized 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 and a modified MM energy of the “real” system, and full geometry optimization is carried out using the gradient of this total energy. Various 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, were examined and compared with the pure ab initio MO and the pure MM results. In most reasonable partition schemes, the IMOMM method can reproduce the pure ab initio and the pure MM geometries and energies quite well.