Two-electron relativistic corrections to the potential energy surface and vibration-rotation levels of water

Two-electron relativistic corrections to the ground-state electronic energy of water are determined as a function of geometry at over 300 points. The corrections include the two-electron Darwin term (D2) of the Coulomb-Pauli Hamiltonian, obtained at the cc-pVQZ CCSD(T) level of theory, as well as th e Gaunt and Breit corrections, calculated perturbationally using four-compo nent fully variational Dirac-Hartree-Fock (DHF) wavefunctions and two diffe rent basis sets. Based on the calculated energy points, fitted relativistic correction surfaces are constructed. These surfaces are used with a high-a ccuracy ab initio nonrelativistic Born-Oppenheimer (BO) potential energy hy persurface to calculate vibrational band origins and rotational term values for (H2O)-O-16. The calculations suggest that these two-electron relativis tic corrections, which go beyond the usual kinetic relativistic effects and which have so far been neglected in rovibrational calculations on light ma ny-electron molecular systems, have a substantial influence on the rotation -vibration levels of water. The three effects considered have markedly diff erent characteristics for the stretching and bending levels, which often le ads to fortuitous cancellation of errors. The effect of the Breit interacti on on the rovibrational levels is intermediate between the effect of the ki netic relativistic correction and that of the one-electron Lamb-shift effec t. (C) 2001 Elsevier Science B.V. All rights reserved.