Oleg L. Polyansky

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

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 morbid 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 morbid 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 po...

Supplementary material from "High-accuracy water potential energy surface for the calculation of infrared spectra"

Transition intensities for small molecules such as water and CO 2 can now be computed with such high accuracy that they are being used to systematically replace measurements in standard databases. These calculations use high-accuracy ab initio xa0dipole moment surfaces and wave functions from spectroscopically determined potential energy surfaces (PESs). Here, an extra high-accuracy PES of the water molecule (H 2 16 O) is produced starting from an ab initio xa0PES which is then refined to empirical rovibrational energy levels. Variational nuclear motion calculations using this PES reproduce the fitted energy levels with a standard deviation of 0.011u2009cm −1 , approximately three times their stated uncertainty. The use of wave functions computed with this refined PES is found to improve the predicted transition intensities for selected (problematic) transitions. A new room temperature line list for H 2 16 Oxa0is presented. It is suggested that the associated set of line intensities is the most accurate available to date for this species.This article is part of the theme issue ‘Modern theoretical chemistry’.