On the application of the IMOMO (integrated molecular orbital plus molecular orbital) method

Five years ago Morokuma and colleagues introduced the IMOMO method, which i ntegrates two molecular orbital methods into one calculation. Since then, t he method has been expanded in several ways; it has been generalized to con sider up to three methods, and has been unified as the ONIOM method to incl ude both MO and MM combinations. In this review we present the history of t he method, a number of chemical problems that we have studied, how to asses s IMOMO combinations and partitionings, and our latest efforts that take th e method beyond the conventional investigation of ground state energy surfa ces. In particular, we emphasize the importance of the S-value test for val idation of the ONIOM method/model combinations. The method combination depe nds much on the properties and accuracies required. Generally speaking, how ever, if the target level is CCSD(T) or G2, the best choice of low level is MP2. If MP2 or DFT is the target level, HF or eventually semiempirical MO methods are good choices of low level. These methods can be further combine d with an outer-most layer of the MM level. (C) 2000 John Wiley & Sons, Inc .