Multi-state empirical valence bond

A second important application of CMD has been to study the dynamics of the hydrated proton. This study involved extensive CMD simulations to determine the proton transport rate in on our Multi-State Empirical Valence Bond (MS-EVB) model for the hydrated proton. = Shown in Fig. 4 are results for the population correlation function, (n(t)n(O)), for the Eigen cation, HsO, in liquid water. Also shown is the correlation function for D3O+ in heavy water. It should be noted that the population correlation function is expected to decay exponentially at long times, the rate of which reflects the excess proton transport rate. The straight line fits (dotted lines) to the semi-log plots of the correlation functions give this rate. For the normal water case, the CMD simulation using the MS-EVB model yields excellent agreement with the experimental proton hopping... 

An important contribution from the ADMP treatment of water clusters has been the so-called amphiphilic nature, or directional hydrophobicity, of the hydrated proton. Through ADMP treatment of protonated water clusters [24], and through the study of large water-vacuum interface systems [25] using the computationally efficient second generation Multi-State Empirical Valence Bond (MS-EVB2) approach [186,187] it was demonstrated that a protonated species tends to reside on the surface of a water vacuum interface with its lone pairs directed away from the neighboring water molecules. These results have subsequently been confirmed by many experimental [188-191] and theoretical studies [192,193]. 

Cuma, M., Schmitt, U.W. and Voth, G.A. (2001). A multi-state empirical valence bond model for weak acid dissociation in aqueous solution. J. Phys. Chem. A 105 (12), 2814-2823... 

As an example for organic liquids and proton transport therein we will discuss liquid imidazole. As the above-mentioned phosphonic acid, immobilised imidazole can be used in applications, such as fuel-cell membranes. Early studies have shown the different time scales that are involved in proton-transfer mechanisms. Recently, Chen and co-workers have used a multi-state empirical valence-bond (MS-EVB) model to atomistically simulate proton transport in liquid imidazole. The system consists of 216 imidazole molecule and one excess proton. 

The multi-state empirical valence bond (MS-EVB) model elaborated by Voth and his group is more flexible since the valence bond functions are optimized at each step of the molecular dynamics calculation.This model is well adapted for the study of diffusion processes and proton transport chains."... 

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