IMPORTANCE OF RELATIVISTIC CORRECTIONS TO THE SPECTRUM OF WATER AND HYDROGEN SULPHIDE

[1] J. Tennyson, P. Barletta, M.A. Kosten, O.L. Polyansky and N.F. Zobov, Spectrochimica Acta A 58 (2002) 663. [2] H.M. Quiney, P. Barletta, G. Tarczay, A.G. Cs\'{a}sz\'{a}r, O.L. Polyansky and J.Tennyson, Chem. Phys. Lett. 344 (2001) 413.Author Institution: Department of Physics and Astronomy, University College London; Department of Theoretical Chemistry, E\""{o}tv\""{o}s UniversityThe importance of effects as small as minor relativistic corrections in theoretical predictions of energy levels can be as high as few wavenumbers [1]. Two-electron relativistic corrections to the ground-state electronic energy of water and hydrogen sulphide are determined at over 300 (250) geometries. The corrections include the two-electron Darwin term of the Coulomb-Pauli Hamiltonian, and the Gaunt and Breit corrections, calculated perturbationally using four-component variational Dirac-Hartree-Fock wavefunctions. Fitted relativistic correction surfaces are constructed and used with an accurate ab initio nonrelativistic Born-Oppenheimer potential to calculate vibrational and rotational levels. The calculations suggest that these two-electron relativistic corrections, which have so far been neglected in rovibrational calculations on light molecules, have a substantial influence on the ro-vibrational levels [2]. The effects considered have markedly different characteristics for the stretching and bending levels