The adiabatic correction for nonlinear triatomic molecules: Techniques and calculations

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Abstract
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.