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

Five years ago Morokuma and colleagues introduced the IMOMO method, which integrates two molecular orbital methods into one calculation. Since then, the method has been expanded in several ways; it has been generalized to consider up to three methods, and has been unified as the ONIOM method to include both MO and MM combinations. In this review we present the history of the method, a number of chemical problems that we have studied, how to assess IMOMO combinations and partitionings, and our latest efforts that take the method beyond the conventional investigation of ground state energy surfaces. In particular, we emphasize the importance of the S-value test for validation of the ONIOM method/model combinations. The method combination depends much on the properties and accuracies required. Generally speaking, however, 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 combined with an outer-most layer of the MM level. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1419–1432, 2000