Generalized diatomics-in-molecules theory

Abstract

The diatomics-in-molecules hamiltonian has been cast into the most general form composed of fragment hamiltonians, interatomic interaction potential energy terms and arbitrary functions of molecular geometry with adjustable parameters. Based on this hamiltonian, a generalized diatomics-in-molecules (GDIM) method within the framework of semiempirical valence bond theory is proposed as a means for fitting potential energy surfaces. The method requires evaluation of interactomic interaction energies in terms of molecular integrals and utilizes the geometry functions as a calibration device. A preliminary application to the H₃ system is reported. With only one fitting parameter in the entire treatment, the GDIM potential energy surface has a classical barrier height in agreement with the most accurate ab initio CI value and, for linear symmetric points in the significant range of H-H distances, it has a root-mean-square deviation of 0·0012 hartree from the most accurate ab initio energies. An extension of the GDIM method to a method of molecules-in-molecules is also proposed. The presentation of these methods is timely considering the current wide interest in developing methods for fitting ab initio potential energy surfaces.