S. Humbel, S. Sieber, and K. Morokuma, “The IMOMO method: Integration of different levels of molecular orbital approximations for geometry optimization of large systems: Test for n-butane conformation and SN2 reaction: RCI+Cl-,” J. Chem. Phys., 105 (1996) 1959-67. DOI: 10.1063/1.472065
