A spectroscopically determined potential energy surface for the ground state of H216O: A new level of accuracy

Abstract

The potential energy function for the electronic ground state of the water molecule has been obtained by fitting rotation‐vibration term values involving J≤14 for 24 vibrational states of H₂¹⁶O together with 25 additional vibrational term values belonging to higher excited states. The fitting was carried out by means of an exact kinetic energy Hamiltonian. It was found that the differences between the exact kinetic energy calculations and calculations with the m o r b i d program (i.e., calculations with an approximate kinetic energy operator) depend only very slightly on the parameters of the potential. This fact allowed us to make an inexpensive fitting using the m o r b i d approach and still get the accuracy obtainable with the exact kinetic energy Hamiltonian. The standard deviation for 1600 term values was 0.36 cm⁻¹. For 220 ground state energy levels the standard deviation was 0.03 cm⁻¹. With the fitted potential, calculations of term values with J≤35 were carried out. This showed the excellent predictive power of the new potential. For the J=20 term values in the vibrational ground state, the deviations from experiment are typically below 0.2 cm⁻¹. The discrepancy for the observed level with the highest K_a value, J_KaKc = 20₂₀₀, is only 0.008 cm⁻¹. The calculated term value for the observed level with the highest J, 35₀₃₅, deviates 0.1 cm⁻¹ from experiment. Because of the level of accuracy achieved in these calculations, we can for the first time demonstrate the breakdown of the Born–Oppenheimer approximation for the water molecule. The high K_a level calculations allow us to show that the rotational energy level structure in water is at least of a very different nature than the fourfold cluster structures observed for H₂Se and calculated for H₂S, H₂Se, and H₂Te.