One‐ and Two‐Center Expansions of the Breit‐Pauli Hamiltonian

The orbit‐orbit, spin‐spin, and spin‐orbit Hamiltonians of the Breit‐Pauli approximation are expressed in terms of irreducible tensors. One‐ and two‐center expansions are given in a form in which the coordinate variables of the interacting particles are separated. In the one‐center expansions of the orbit‐orbit and spin‐orbit Hamiltonians the use of the gradient formula reduces some of the infinite sums to finite ones. Two‐center expansions are discussed in detail for the case of nonoverlapping charge distributions. The angular parts of the matrix elements of these Hamiltonians are evaluated for product wavefunctions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70736/2/JMAPAQ-9-9-1357-1.pd

An Exchange-Coulomb Model Potential Energy Surface for the Ne-CO Interaction. I. Calculation of Ne-CO van der Waals Spectra

Exchange-Coulomb model potential energy surfaces have been developed for the Ne–CO interaction. The initial model is a three-dimensional potential energy surface based upon computed Heitler–London interaction energies and literature results for the long-range induction and dispersion energies, all as functions of interspecies distance, the orientation of CO relative to the interspecies axis, and the bond length of the CO molecule. Both a rigid-rotor model potential energy surface, obtained by setting the CO bond length equal to its experimental spectroscopic equilibrium value, and a vibrationally averaged model potential energy surface, obtained by averaging the stretching dependence over the ground vibrational motion of the CO molecule, have been constructed from the full data set. Adjustable parameters in each model potential energy surface have been determined through fitting a selected subset of pure rotational transition frequencies calculated for the 20Ne-12C12O isotopolog to precisely known experimental values. Both potential energy surfaces provide calculated results for a wide range of available experimental microwave, millimeter-wave, and midinfrared Ne–CO transition frequencies that are generally far superior to those obtained using the best current literature potential energy surfaces. The vibrationally averaged CO ground state potential energy surface, employed together with a potential energy surface obtained from it by replacing the ground vibrational state average of the CO stretching dependence of the potential energy surface by an average over the first excited CO vibrational state, has been found to be particularly useful for computing and/or interpreting mid-IR transition frequencies in the Ne–CO dimer