Examining the Role of Different Molecular Interactions on Activation Energies and Activation Volumes in Liquid Water

Abstract

There are a large number of force fields available to model water in molecular dynamics simulations, which each have their own strengths and weaknesses in describing the behavior of the liquid. One particular weakness in many of these models is their description of dynamics away from ambient conditions, where their ability to reproduce measurements is mixed. To investigate this issue, we use the recently developed fluctuation theory for dynamics to directly evaluate measures of the local temperature and pressure dependence: the activation energy and the activation volume. We examine these activation parameters for hydrogenbond jump exchange times, OH reorientation times, and diffusion coefficients calculated from the SPC/E, SPC/Fw, TIP3P-PME, TIP3P-PME/Fw, OPC3, TIP4P/2005, TIP4P/Ew, E3B2, and E3B3 water models. Activation energy decompositions available through the fluctuation theory approach provide mechanistic insight into the origins of different temperature dependences between the various models, as well as the influence of three-body effects and flexibility.

Using Activation Energies to Elucidate Mechanisms of Water Dynamics

“…37,38 For reference, the measured diffusion coefficient 12 is 2.30 × 10 −5 cm 2 /s and the experimental reorientation time 39,40 is 2.6 ps; the agreement of the water models with these measured values has been examined in detail previously. 17 ) In both cases, the effective rate constants, i.e., D and 1/τ 0 as well as 1/τ 2 and 1/τ 0 , are nearly perfectly linearly related, consistent with the assumption that the diffusive and reorientation dynamics are determined by H-bond jumps. However, it is interesting to note that for the OH reorientation time, Fig.…”

“…We have recently used this approach to examine many dynamical and equilibrium properties of water [16][17][18][19][20][21][22][23][24][25][26] (and other systems [27][28][29] ) including OH reorientation. [17][18][19][20][21] and self-diffusion. 17,20,21,24 One can use these results to carry out a simple thought experiment.…”

“…[17][18][19][20][21] and self-diffusion. 17,20,21,24 One can use these results to carry out a simple thought experiment. If OH reorientation and diffusion (and other dynamics) in water is controlled by H-bond exchanges, the jump time should be the single characteristic of all of these dynamics.…”

“…[55][56][57] In the case of water, the pressure dependences of dynamical properties such as the diffusion coefficient and the integrated reorientation time (the integral of C 2 (t) accessible from NMR) have been examined. 17,24,43,44,58,59 These prop-erties are not "Arrhenius" in pressure -i.e., ln k is not linear in p so that ∆V ‡ changes significantly with pressure -and thus activation volumes are typically determined by measuring a timescale at a large number of pressures and fitting the results to a functional form. 43,44,58,59 At the same time, even relatively small changes in the timescale usually require increasing the pressure by hundreds or thousands of bar, which has limited the attention of simulation studies.…”

“…The fluctuation theory for dynamics approach enables the calculation of the activation volume from simulations at a single pressure. 17,24 In analogy with eq. 9, the activation volume for the diffusion coefficient is given by…”