Electronic structure and bond length dependence of the effective valence shell Hamiltonian of S_{2} as studied by quasidegenerate manybody perturbation theory
Abstract
The effective valence shell Hamiltonian (H^{v}) of S_{2} is calculated as a function of internuclear distance using quasidegenerate manybody perturbation theory with the full valence space spanned by eight valence orbitals. Calculated potential curves and excitation energies for several valence states are in good agreement with experiment and are compared with configuration interaction calculations using the same primitive basis. In order to test assumptions of semiempirical theories, we also perform a more approximate calculation of H^{v} in which the valence space is constructed as the union of the atomic valence spaces with the atomic orbitals taken from atomic SCF calculations. A new and important feature of this approximate, correlated H^{v} is the use of optimized valence and excited orbitals as determined from a constrained SCF procedure. The matrix elements of this approximate, correlated H^{v} are transformed to the original nonorthogonal atomic valence basis, and their bond length dependences are fit with simple analytical functions. Some calculated H^{v} matrix elements agree with the forms commonly postulated for semiempirical integrals, while others display quite different behavior. An example of the latter are the onecenter, twoelectron integrals which depend significantly on bond length in marked contrast to semiempirical theories which assume them to be bond length independent.
 Publication:

Journal of Chemical Physics
 Pub Date:
 March 1987
 DOI:
 10.1063/1.452753
 Bibcode:
 1987JChPh..86.2899W