Full-dimensional, ab initio potential energy and dipole moment surfaces for water

Abstract

We report full-dimensional, ab initio potential energy (PES) and dipole moment surfaces (DMS) for water. The PES is a sum of one-, two- and three-body terms. The three-body potential is a fit, reported here, to roughly 30,000 intrinsic three-body energies obtained with second-order Møller-Plesset perturbation theory (MP2) and using the aug-cc-pVTZ basis set (avtz). The one- and two-body potentials are from an ab initio water dimer potential [Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The predictive accuracy of the PES is demonstrated for the water trimer, tetramer, and hexamer by comparing the energies and harmonic frequencies obtained from the PES and new high level ab initio calculations at the respective global minima. The DMS is constructed from one- and two-body dipole moments, based on fits to MP2/avtz dipole moments. It is shown to be very accurate for the hexamer by comparison with direct calculations of the hexamer dipole. To illustrate the anharmonic character of the PES one-mode calculations of the 18 monomer fundamentals of the hexamer are reported in normal coordinates.

Three-dimensional infrared spectroscopy of isotope-diluted ice Ih

“…8b) or when employing a parameterized potential by Bowman and coworkers (label ii - Fig. 8c) [66,67] . When placing the dimer into a crystal environment, which adds both electronic and orientational polarizability that could in principle stabilize (solvate) the auto-ionized state, this feature becomes a bit more pronounced (label iii - Fig.…”

“…To that end, we considered a water dimer (H 2 O) 2 both in vacuum and in the environment of an ice Ih crystal. In the vacuum case, we calculated the potential energy surface on the MP2/aug-cc-pVTZ level of theory [65] and using a parameterized potential made available by Bowman and coworkers [66,67], which has been fitted to CCSD(T)/aug-cc-pVTZ calculations (that potential was also recently applied to simulate the FT-IR spectra of neat ice [68]). We started from a minimized configuration of the water dimer and varied the r OD and R O···O distances, keeping all other coordinates fixed.…”

Concerted hydrogen-bond breaking by quantum tunneling in the water hexamer prism

“…The cluster dynamics can be probed by high-resolution rovibrational spectroscopy (1)(2)(3)(4)(5)(6)(7)(8)(9) and interpreted by theoretical simulations (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27). The nature of the interactions between the water molecules is the same in the clusters as in the bulk (many-body forces beyond the three-body term are relatively weak) (28), and hence the cluster spectra can be used to test universal models (25,(29)(30)(31)(32) of the water intermolecular potential energy surface, giving insight into hydrogen bonding in all phases of water. At low temperatures, the molecules are frozen into a network and can only rearrange by quantum tunneling, which causes splittings in the spectrum ranging from MHz to THz (33).…”

Predictions for water clusters from a first-principles two- and three-body force field

“…Clearly, our hybrid approach is more than adequate at the current accuracy level of water potentials. For comparison, the calculations of Wang et al 46,47 used the aTZ basis set at the MP2 level. Results at this level of theory are also shown in Table I.…”

“…Generation of such fits would require calculations of 2.1 × 10 6 data points using a mere 2 points per dimension and as many as 10 × 10 9 data points with 3 points per dimension. The flexible-monomer three-body potentials of Wang et al, fitted to 30 000 points 46 or 40 000 points, 47 use only about 1.6 points per dimension. For the six-dimensional rigid-monomer water dimer, this number of points per dimension would result in a total of only 20 grid points, clearly an inadequate number.…”