Properties and 3D Structure of Liquid Water:  A Perspective from a High-Rank Multipolar Electrostatic Potential

We propose a new rigid, nonpolarizable high-rank multipolar potential for the simulation of liquid water. The electrostatic interaction is represented by spherical tensor multipole moments on oxygen and hydrogen, up to hexadecupole. The Quantum Chemical Topology (QCT) method yields the atomic multipole moments from a MP2/aug-cc-p-VTZ electron density of a single water molecule in the gas phase. These moments reproduce the experimental molecular dipole and quadrupole moment within less than 1%. Given its high-rank multipole moments, used in conjunction with a consistent high-rank multipolar Ewald summation, the QCT potential is ideal to assess the performance of exhaustive "gas phase" electrostatics in molecular dynamics simulations of liquids. The current article explores the performance of this potential at 17 temperatures between -35 °C (238 K) and 90 °C (363 K) and at 7 pressures between 1 and 10 000 atm. The well-known maximum in the liquid's density at 4 °C is reproduced at 6 °C. Six bulk properties are calculated and found to deviate from experiment in a homogeneous manner, that is, without serious outliers, compared to several other potentials. Spatial distribution functions (i.e., gOO(r,Ω)) and the (more common) radial distribution functions are used to analyze the local water structure. At the lone pair side of a central water, neighboring waters form a continuous horseshoe-like distribution, with substantial narrowing in the central part. The latter feature is unique to the QCT potential. Under high pressure, the local structure undergoes dramatic rearrangement and results in the collapse of second shell neighbors into the interstitial region of the first shell, which is in close agreement with experiment. Our results also corroborate the suggestion that the local hydrogen-bonded network remains largely intact even under such conditions.