Dynamic simulations, for aqueous solutions

An extension of the molecular dynamics simulation to study aggregation of H0O solutes in water was carried out by Wallqvist (1991a,b). Rappaport and Scheraga (1982) found no evidence of aggregration of non-polar solutes in water-like solvent. Recently, Paschek (2004) carried out extensive molecular dynamics simulations of aqueous solutions using various models for water-like particles. 

One of the main advantages of the stochastic dynamics methods is that dramatic tirn savings can he achieved, which enables much longer stimulations to he performed. Fc example, Widmalm and Pastor performed 1 ns molecular dynamics and stochastic dynamic simulations of an ethylene glycol molecule in aqueous solution of the solute and 259 vvatc jnolecules [Widmalm and Pastor 1992]. The molecular dynamics simulation require 300 hours whereas the stochastic dynamics simulation of the solute alone required ju 24 minutes. The dramatic reduction in time for the stochastic dynamics calculation is du not only to the very much smaller number of molecules present hut also to the fact the longer time steps can often he used in stochastic dynamics simulations. 

We now develop an example of this variational character. We utilize results from ab initio molecular dynamics (AIMD) for that purpose, and estimate fiquid water. The ab initio molecular dynamics simulations were carried out with the VASP (Kresse and Hafner, 1993 Kresse and Furthmiiller, 1996) simulation program, as described in detail in Asthagiri et al. (2003c). Ab initio molecular dynamics of aqueous solutions are recent activities compared with other simulation methods for aqueous solutions, and basic characterization of the new methods is still underway see Grossman et al. (2004) and Schwegler et al. (2004) for initial examples. 

Tongraar et al. ° studied N03 in an aqueous solution using QM/MM molecular dynamics simulations for a system consisting of one N03 ion and 199 water molecules. They found that the oxygen atoms of the N03 ion participate in weak hydrogen bonds to the water molecules that easily are broken and formed. 

A promising method, developed in recent years, is the use of first principles molecular dynamics as exemplified by the Car-Parrinello technique (8]. In these calculations the interatomic potentials are explicitly derived from the electronic ground-state within the density functional theory in local or non-local approximation. It combines quantum mechanical calculations with molecular dynamics simulations and, therefore, overcomes the limitations of both methods. Actual computers allow only simulations of aqueous solutions of about 60 water molecules for several ps (10 s). This limit is still at least one order of magnitude shorter than the fastest directly measured water exchange rate, k = 3.5 x 10 s for [Eu(H20)8], i.e. one exchange event every (8 x 3.5 x lO s ) = 36 ps [9]. Nevertheless, several publications appeared in the late 1990s on solvated Be [10], K+ [11] and Cu + [12] presenting mainly structural results. 

For most electrolyte solutions at or near room temperature, there have been two main approaches, exemplified by the Pitzer and HKF models to be described below. (These two approaches were also mentioned on page 304.) In one, referred to as ion-interaction theory and embodied in the Pitzer equations, no attempt is made (except if there are very strong complexes present) to identify species. Components are treated stoichiometrically, and all ion interactions are accounted for in the form of fit coefficients in some form of equation. Advocates of this approach point to the rather uncertain nature of our knowledge of aqueous species. Molecular dynamics simulations of these solutions... 

Results with a certain degree of reliability from MD simulations of aqueous solutions reported up to now are restricted to structural properties of such solutions. In the section on aqueous solutions below very preliminary velocity autocorrelation functions are calculated from an improved simulation of a 9.55 molal NaCl solution. The problem connected with the stability of the system and the different cut-off parameters for ion-ion, ion-water and water-water interactions are discussed. Necessary steps in order to achieve quantitative results for various dynamical properties of aqueous electrolyte solutions are considered. 

Calculations for liquid water and also for aqueous solutions have been performed by other methods. Rahman and Stillinger (1971, 1973), Rahman et ah (1975), and Stillinger and Rahman (1972, 1974a,b) used molecular dynamics simulation techniques. Owicki and Scheraga (1977a,b,c) have pro-... 

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