A new ab initio ground-state dipole moment surface for the water molecule

Abstract

A valence-only ͑V͒ dipole moment surface ͑DMS͒ has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation ͑C͒ and relativistic ͑R͒ effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H 2 16 O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539Ϯ 0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546͑6͒ D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge ͓J. Chem. Phys. 113, 16 ͑2000͔͒. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces ͑PESs͒ has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions.

Global spectroscopy of the water monomer

“…The resulting DMS reproduces the intensities of high-precision measurements [75] to better than 1 per cent, and to better than 10 per cent for 20 000 strong and medium-strength lines from HITRAN [77]. These results are a significant improvement on the published DMSs of Schwenke & Partridge [15] and Lodi et al [69]. This new DMS paves the way for accurate calculation of the global line list of water isotopologues.…”

“…As corrections for high-order correlation effects and relativistic correction have become incorporated into the PES [12] and the QED correction is likely to be at least an order of magnitude smaller than these, removing the remaining discrepancy between experiment and theory must rely on better treatments of the adiabatic and non-adiabatic corrections to the BO approximation and inclusion of spin-orbit coupling effects [65]. As pointed out by Császár et al [12], the existing high-accuracy DMSs [15,69] are also inadequate for the calculation of intensities of bands at dissociation. This point is addressed in §6, where a new, much more accurate DMS suitable for the intensity calculations at dissociation is discussed.…”

“…There are very high-quality DMSs published in the literature [15,69]; however, at least two issues remain unsolved with these DMSs. They were not aimed for global spectroscopy and their predicted line intensities were accurate to only about 2-3 per cent.…”

“…The wave functions of these levels were calculated using the DVR3D nuclear motion code with the fitted PES12 PES [37]. These wave functions were used as input data for a program computing line strengths using both the published CVR DMS [69] and our new DMS given by Lodi et al [76]. The line strengths, together with line positions, are used to obtain the calculated values of the Stark coefficients.…”

Perspective: Accurate ro-vibrational calculations on small molecules

“…The Hellmann-Feynman theorem shows that these two methods are equivalent for exact wavefunctions. High accuracy tends to favor the use of finite differences despite the extra computational cost: the finite difference method is more accurate but, perhaps more importantly, as an energy-based method it allows small corrections to the DMS to be introduced along the lines of the FPA procedure used for PES 123,124 even for corrections which use first-order perturbation theory.…”