The IMOMO method: Integration of different levels of molecular orbital approximations for geometry optimization of large systems: Test for <i>n</i>-butane conformation and <i>S</i> <i>N</i>2 reaction: RCl+Cl−

A new theoretical method, called IMOMO (integrated MO (molecular orbital)+MO), for integration of two different levels of MO approximation is presented. Only the active or more difficult part of a molecule is treated at a higher level of approximation and the rest of the molecule at a lower level of approximation. The integrated total energy and energy derivatives are defined from three different calculations, and the structure of transition state as well as the equilibrium structure can be optimized using the integrated energy. Any combination of any molecular orbital approximations (ab initio, density functional to semi-empirical) can be used. Test calculations in the IMOMO method have been performed and compared with normal MO calculations for the conformation energy of ethane and n-butane and the SN2 reaction of ethyl, propyl, isobutyl, and neopentyl chloride with Cl−. The results indicate that these methods have a tremendous potential for theoretical study of larger molecules, in particular for transition states.