PNNL

Abstract

We assess the performance of 7 pairwise additive (TIP3P, TIP4P, TIP4P-ice, TIP5P, OPC, SPC, SPC/E) and 6 families of many-body potentials (AMOEBA, EFP, TTM, WHBB, MB-pol, MBUCB) in reproducing high-level ab initio benchmark values (CCSD(T) or MP2) for the binding energy and the many-body expansion (MBE) of water clusters n = 2-11, 16-17, 20, 25. By including a large range of cluster sizes having dissimilar hydrogen bonding networks, we obtain an understanding of how these potentials perform on different hydrogen bonding arrangements that are mostly outside of their parameterization range. While it is appropriate to compare the results of ab-initio based many-body potentials directly to the electronic binding energies (De’s), the pairwise additive ones are rather compared to the enthalpies at T = 298 K, ???(298??), as the latter class of force fields are parametrized to reproduce enthalpies (implicitly accounting for zero-point energy corrections) rather than binding energies. We find that all pairwise additive potentials considered overestimate the reference ??? values for the n = 2-25 clusters by >17%. For the water dimer (n = 2) in particular, the errors are in the range 86 – 123% for the pairwise additive potentials studied since these are based on an effective rather than the true 2-body interaction as a means of partially accounting for the missing many-body terms. This stronger 2- body interaction is achieved by an enhanced monomer dipole moment that mimics its increase from the gas phase monomer to the condensed phase value. Indeed, for cluster sizes n ³ 4 the percent deviations become slightly smaller (albeit all exceeding 17%). In contrast, we find that the many-body potentials perform more accurately in reproducing the binding energies (De) throughout the entire cluster range (n = 2-25), all reproducing the ab-initio benchmark binding energies within ±7% of the respective CBS values. We further assess the ability of a subset of the many-body potentials (MB-UCB, MB-pol, and TTM2.1-F) to reproduce the magnitude of the ab initio many-body energy terms for water cluster sizes n = 7, 10, 16 and 17. The potentials show an overall good agreement with the benchmark values, mostly exhibiting errors in the 1- and the 3-body terms. Both these terms tend to be underestimated with MB-UCB yielding the best 3-body term, MB-pol most closely reproducing the 1-body term and TTM2.1-F best reproducing the overall binding energies albeit by a cancellation of errors between the 1- and 3- body terms. In an attempt to draw a parallel between the pairwise additive and many-body potentials, we report the analysis of the individual molecular dipole moments for water clusters with 1 to ~4 solvation shells with the TTM2.1-F potential. We find that the internally solvated 3 water molecules have in general a larger molecular dipole moment ranging from 2.6 – 3.0 D. This justifies the use of an enhanced, with respect to the gas phase value, molecular dipole moment for the pairwise additive potentials, thus attempting to fold in the many body terms through the choice of the charges. These results have important implications for the development of future generations of efficient and highly accurate classical interaction potentials in both the pairwise additive and many-body categories.