RELATIVISTIC CORRECTION TO THE POTENTIAL-ENERGY SURFACE AND VIBRATION-ROTATION LEVELS OF WATER

The relativistic correction to the electronic energy of the water mole cule is calculated as a function of geometry using CCSD(T) wavefunctio ns and first-order perturbation theory applied to the one-electron mas s-velocity and Darwin terms. Based on the calculated 324 energy points , a fitted relativistic correction surface is constructed. This surfac e is used with a high-accuracy ab initio non-relativistic Born-Oppenhe imer potential energy surface to calculate the vibrational band origin s and rotational term values for (H2O)-O-16. These calculations sugges t that the relativistic correction, has a stronger influence on the vi bration-rotation levels of water than the Born-Oppenheimer diagonal co rrection. The effect is particularly marked for vibrational levels wit h bending excitation or rotational states with high K-a, (C) 1998 Else vier Science B.V. All rights reserved.