Theoretical dipole moment functions of the HF, HCl, and HBr molecules

Abstract

Potential energy and dipole moment functions of the HF, HCl, and HBr molecules in their electronic ground states have been calculated from highly correlated SCEP/CEPA ab initio wave functions. Purely rotational transition energies are obtained with an accuracy of about 0.1 cm−1, and vibrational transition energies agree within 10–20 cm−1 with the experimental values. The SCEP/CEPA dipole moments in the vibrational ground states are calculated to be (experimental values in parenthesis) 1.807 D (1.826 D) for HF, 1.120 D (1.1085 D) for HCl and 0.829 D (0.828 D) for HBr. For HF various theoretical approaches, i.e., the SCEP/VAR (including variationally all singly and doubly excited configurations), SCEP/CEPA (accounting approximately for unlinked cluster effects), and MC-SCF (with eight optimized valence configurations and with 66 configurations including atomic correlation) methods are compared. The spectroscopic constants and dipole moment functions calculated from SCEP/CEPA and MC-SCF wave functions are of comparable accuracy. The SCEP/CEPA and MC-SCF dipole moment functions of HF are in good agreement with the experimental function over a range of internuclear distances which covers approximately the nine lowest vibrational states. The theoretical potential and dipole moment functions have been used to calculate vibrational dipole matrix elements. The fully ab initio results of HF and HCl up to v=5 agree within about 5% with the values derived from experiments. For HBr strongly differing slopes of the dipole moment function have been reported in the literature. The present theoretical results are in good agreement with the most recent measurements and enable a reliable estimate of the absolute intensity for the 0–1 vibrational transition.