Application of effective hamiltonian theory to the method of diatomics-in-molecules

It is shown how effective hamiltonian theory can be applied to the method of diatomics-in-molecules (DIM) at two different levels: (1) in the construction of the input diatomic fragment matrices, especially when a block-diagonal matrix is required, and (2) in the definition of diabatic states for the polyatomic system. The former application is a special case of the previous procedure used in DIM and can be applied whenever intruder states are not important.