Adiabatic correction for nonlinear triatomic molecules: Techniques and calculations

In the Born-Oppenheimer (BO) approximation the isotopic-mass-independent BO electronic energy is the potential energy surface for nuclear motion. The first correction to the BO approximation is obtained by adding the isotopic-mass-dependent adiabatic correction C to this potential energy surface. The adiabatic correction for a nonlinear triatomic molecule in a nondegenerate electronic state is expressed in terms of one-electron integrals over atomic basis functions. Both single determinant molecular orbital BO electronic wavefunctions and configuration interaction wavefunctions are discussed. The adiabatic correction is calculated for H2O and H+3 and their isotopic variants at the respective equilibrium internuclear geometries. Theses adiabatic corrections are combined with those previously calculated for diatomic molecules to obtain correction factors to equilibrium constants for H/D isotopic exchange reactions calculated within the framework of the BO approximation