A second generation multistate empirical valence bond model for proton transport in aqueous systems

Abstract

Building on the previously developed multistate empirical valence bond model [U. W. Schmitt and G. A. Voth, J. Chem. Phys 111, 9361 (1999)] for the dynamics and energetics of an excess proton in bulk phase water, a second generation model is described. This model is shown to produce similar dynamic and structural properties to the previous model, while allowing for the use of the full hydronium charge. This characteristic of the model is required for its implementation in a host of realistic applications beyond bulk water. An improved state selection algorithm is also presented, resulting in a significantly reduced energy drift during microcanonical molecular dynamics simulations. The unusually high self diffusion constant of an excess proton in water due to the proton hopping (Grotthuss) process is observed in the simulation data and is found to be quantitatively in the same range as the experimental value if a quantum correction is taken into consideration. Importantly, a more complete analysis of proton transport process is also presented.

Modelling of morphology and proton transport in PFSA membranes

“…Although all of the above ab initio calculations are based on relatively small molecular systems, though in several cases approaching the limits of what can reasonably be performed, very similar conclusions were reached in MD simulations recently conducted by Voth et al 51,52 using both their second-generation (MS-EVB2 45 ) and self-consistent (SCI-MS-EVB 49 ) multi-state empirical valence bond methods on considerably larger molecular systems. In the first of these investigations the simulations were performed on a system consisting of four Nafion chains containing 10 equally spaced monomers, at hydrations where l = 7 and 15.…”

“…However, a more realistic simulation is obtained by using a multi-state EVB (MS-EVB) model in which water molecules in successive hydration shells around the hydrated proton are considered as potential hydrogen bond donors. 45 While the two-state EVB model has the advantage of being relatively computationally inexpensive, allowing the incorporation of multiple protons and polarisible water potentials, the MS-EVB model gives a more correct treatment of proton diffusion, although neither are in quantitative agreement with experimental data on proton diffusion in bulk water. An alternative method for dealing with the inherent quantum mechanical nature of proton transport in PFSA membranes, whilst retaining the ability to simulate a relatively large system, is to embed quantum calculations within a classical molecular mechanics simulation (termed QM/MM techniques 53,54 ) or, more generally, to combine together an arbitrary set of molecular orbital (MO), DFT or molecular mechanics models via the ONIOM method (developed by Morokuma and co-workers, 55,56 and implemented in GAUSSIAN 03).…”

“…In particular, the influence of ''structural diffusion'' on the mobility of protons in the membrane, via the Grothuss mechanism, [39][40][41] is completely neglected by standard classical force fields, and this mechanism is known to be important at high degrees of hydration, or when water is present in confined geometries. 42 Recently, a number of empirical valence bond (EVB) force fields have been produced [43][44][45] with the principal goal of studying proton diffusion in bulk water. [46][47][48][49] These have also been applied to investigate proton diffusion in PFSA systems.…”

Dynamics of water nanodroplets and aqueous protons in non-ionic reverse micelles

“…The main features of this band are two maxima located in the regions of 1900-2200 and 2300-2700 cm À1 . In the case of unconstrained aqueous protons we obtain similar results to those by Voth et al 17 with peaks around 2065 and 2650 wavenumbers. In a previous work 41 we could associate such bands basically to the contribution from Zundel and Eigen-like structures, respectively.…”

“…ðtÞ includes contributions from nuclear velocities and polarization fluctuations. 17,32 To calculate _ m ! ðtÞ, we considered exclusively the six instantaneous valence bond states exhibiting the largest weights c 2 i and the estimates for the time derivatives of the different c 2 i were obtained from standard first order perturbation theories in the form:…”