Quantum chemical computations on parts of large molecules: the ab initio local self consistent field method

In this Letter we extend to the ab initio level the principles of the local self consistent field method (LSCF) developed earlier within the semiempirical approximations. It allows quantum electronic computations on a fragment of a large molecule mostly depicted by a classical force field. The bond separating the quantum subsystem from the classical one is assumed to be represented by localized orbitals introduced as data. The molecular orbitals of this subsystem are developed on a basis of functions, orthogonal to the localized orbitals, which derive from the atomic orbitals of the usual basis set by a simple linear transformation. The algorithm of the LSCF method is obtained by a simple modification of the standard Hartree-Fock or DFT codes.