On the valence bond diatomics-in-molecules method. I. A projection operator reformulation

A rederivation of the diatomics-in-molecules (DIM) method for obtaining potential energy surfaces is presented in terms of projections onto fragment diatomic and atomic subspaces. This approach renders the approximations inherent in the DIM method transparent. The method is considered from both ab initio and semiempirical viewpoints. Nonadiabatic and spin-orbit coupling in the fragments is discussed. Comparison with previous DIM formulations is made.