Low-Order Many-Body Interactions Determine the Local Structure of Liquid Water

Abstract

<div> <div> <div> <p>Despite its apparent simplicity, water displays unique behavior across the phase diagram which is strictly related to the ability of the water molecules to form dense, yet dynamic, hydrogen- bond networks that continually fluctuate in time and space. The competition between different local hydrogen-bonding environments has been hypothesized as a possible origin of the anomalous properties of liquid water. Through a systematic application of the many-body expansion of the total energy, we demonstrate that the local structure of liquid water at room temperature is determined by a delicate balance between two-body and three-body energies, which is further modulated by higher-order many-body effects. Besides providing fundamental insights into the structure of liquid water, this analysis also emphasizes that a correct representation of two-body and three-body energies requires sub-chemical accuracy that is nowadays only achieved by many-body models rigorously derived from the many-body expansion of the total energy, which thus hold great promise for shedding light on the molecular origin of the anomalous behavior of liquid water. </p> </div> </div> </div>

Data-Driven Many-Body Models with Chemical Accuracy for CH4/H2O Mixtures

“…Many-body reference energies, calculated using the SAMBA approach, 122 are listed in Tables 1 and 2 we found to consistently provide good agreement with coupled-cluster reference energies for various aqueous and non-aqueous systems. 83,85,86,96,97,128,129 As expected from the correlation plots shown in Fig. 2, the TTM-nrg PEF displays significantly larger deviations from the ref- Table 2 for individual terms of the MBE in Eq.1 calculated at the ωB97M-V, TTM-nrg, and MB-nrg levels of theory for the same (CH 4 ) m (H 2 O) n clusters, with m + n ≤ 5, shown in Fig.3.…”

“…Similar deficiencies of ωB97M-V in representing 3B energies were also noted in analyses of many-body effects in water. 128 Importantly, since ωB97M-V systematically predicts positive 2B and negative 3B deviations, it appears that this functional benefits from some error compensation between 2B and 3B contributions to the interaction energies in…”

Data-Driven Many-Body Models for Molecular Fluids: CO2/H2O Mixtures as a Case Study

“…It should be noted that our previous analyses showed that, among the existing functionals, ωB97M-V consistently provides the closer agreement with CCSD(T) reference data for molecular interactions in aqueous systems. 69,71,81,82,111 As expected from the analysis of the correlation plots in Fig. 2 A direct probe of the multidimensional 2B energy landscape is provided by the second virial coefficient,…”

“…1 can be correctly represented in terms of classical many-body polarization as described in Section 2.1. In this context, it should be noted that previous studies of manybody effects in aqueous systems indicated that an explicit representation of 3B energies is necessary to guarantee an accurate description of structural, thermodynamic, dynamical and spectroscopic properties of water 75,[109][110][111] as well as halide-water 69,79,[81][82][83][84][85] and alkali-metal ion-water 71,80,86 interactions in the gas phase and in solution. In particular, it was found that significant error cancellation between different terms of the MBE affects the performance of common force fields and DFT models for water.…”

“…In particular, it was found that significant error cancellation between different terms of the MBE affects the performance of common force fields and DFT models for water. 74,109,111,112 To investigate the ability of the TTM-nrg and MB-nrg PEFs to represent many-body effects beyond the 2B term in Eq. 1, we decomposed the interaction energies of the (CO 2 ) m (H 2 O) n clusters, with m+n ≤ 4, shown in Fig.…”