Eq. ( ) can be used as self-consistent field (SCF) equation for equation to determine the best orbitals for the group R interacting with the closed-shell singlets included as one or more groups S, enforcing orthogonality between the orbitals of all groups. Eq. 15 is the starting point of all the developments that go under the name Fragment Ab Initio Model Potential (FAIMP) 19,29 .