Polarizable water potentials

Although a direct comparison between the iterative and the extended Lagrangian methods has not been published, the two methods are inferred to have comparable computational speeds based on indirect evidence. The extended Lagrangian method was found to be approximately 20 times faster than the standard matrix inversion procedure [117] and according to the calculation of Bernardo et al. [208] using different polarizable water potentials, the iterative method is roughly 17 times faster than direct matrix inversion to achieve a convergence of 1.0 x 10-8 D in the induced dipole. 

A partial solution to the problem is offered by the so-called polarizable potentials. As discussed in section III.C, the major component of the three-body nonadditivity for hydrogen-bonded systems are the induction effects. These effects can be modeled by computing the electric field on a given monomer due to other monomers and finding the dipole moments induced by this field. The polarizable water potentials are able to predict the dimer properties much... 

Table 9 Recent Polarizable Water Potentials Used in Simulations...

Brodholt J, Sampoli M, Vallauri R (1995) Parameterizing polarizable intermolecular potentials for water with the ice 1 h phase. Mol Phys 85(l) 81-90... 

Stern HA, Rittner F, Berne BJ, Friesner RA (2001) Combined fluctuating charge and polarizable dipole models application to a five-site water potential function. J Chem Phys 115(5) 2237-2251... 

Although the potential energy functions can be made to reproduce thermodynamic solvation data quite well, they are not without problems. In some cases, the structure of the ion solvation shell, and in particular the coordination number, deviates from experimental data. The marked sensitivity of calculated thermodynamic data for ion pairs on the potential parameters is also a problem. Attempts to alleviate these problems by introducing polarizable ion-water potentials (which take into account the induced dipole on the water caused by the ion strong electric field) have been made, and this is still an active area of research. 

Dang, L.X., Development of nonadditive intermolecular potentials using molecular-dynamics -solvation ofLi+ and F- ions in polarizable water. J. Chem.Phys. (1992) 96 6970-6977. 

Polarizable Intermolecular Potentials for Water with the Ice Ih Phase. 

Combined Fluctuating-Charge and Polarizable Dipole Models Application to a Five-Site Water Potential Function. 

In general, for each acid HA, the HA-(H20) -Wm model reaction system (MRS) comprises a HA (H20) core reaction system (CRS), described quantum chemically, embedded in a cluster of Wm classical, polarizable waters of fixed internal structure (effective fragment potentials, EFPs) [27]. The CRS is treated at the Hartree-Fock (HF) level of theory, with the SBK [28] effective core potential basis set complemented by appropriate polarization and diffused functions. The W-waters not only provide solvation at a low computational cost they also prevent the unwanted collapse of the CRS towards structures typical of small gas phase clusters by enforcing natural constraints representative of the H-bonded network of a surface environment. In particular, the structure of the Wm cluster equilibrates to the CRS structure along the whole reaction path, without any constraints on its shape other than those resulting from the fixed internal structure of the W-waters. 

Beside the empirical or semiempirical models described above, the need for inclusion of many-body effects, polarizability at least, in water-water potentials has also been recognized in the development of more recent ab initio potentials [45,75,103-108]. 

Irrespective of the model, effective or polarizable, adopted to describe water-water interactions, the drawbacks of two-body cation-water potentials derived by ab initio calculations are well known. They become especially serious with di- and trivalent cations, i.e. when the electric field the ion produces is more intense. 

Our calculations can be viewed as an extension of the work of Bryk and Haymet."" They studied the behavior of the Na and Cf ions at the static ice/water interface on the nanosecond timescale. We were interested in the solidification process itself and in the expulsion of the ions into the remaining liquid. This required using longer simulation times (hundreds of nanoseconds). For the purpose of our research, we used the SPC/E water potential.Potential parameters for sodium and chloride ions were taken as the non-polarizable set from Ref. 41. 

The most important component of the water trimer nonadditive energy is the induction interaction of the second order in V. Its simple mechanism is shown in Fig. 33.3 a permanent multipole moment on monomer A induces multipole moments on monomer B which in turn interact with the permanent multipole moments of monomer C. Higher orders involve interactions between induced moments. The nonadditive induction energy is in general the most important nonadditive component for hydrogen-bonded systems. As already mentioned, it is the only term used—and only in the asymptotic approximation, i.e. neglecting charge-overlap effects—in the polarizable empirical potentials. 

FIG. 1 Typical electrochemical cell employed for electrochemical studies at the polarizable water] 1,2-dichloroethane interface. Cyclic voltammograms in the presence of various supporting electrolytes are also illustrated. Polarizable potential windows close to 1 V can be obtained employing Li2S04 and BTPPA TPFB as supporting electrolytes in the aqueous and organic phase, respectively. (From Ref. 6.)... 

More recently, Samec and coworkers investigated the line shape of the fluctuation spectrum at the polarizable water/DCE interface in the presence of the phospholipid DL-a-dipahnitoyl-phosphatidylcholine (DPPC) [32]. The line shape of experimental power spectra similar to those exemplified in Pig. 4.12 b was analyzed in terms of the mean vertical displacement of the interface generated by capillary waves. The experimental results in the presence and absence of DPPC at a wide potential range appear consistent with the description of the liquid/liquid boundary as molecularly sharjf. However it is not entirely clear from this analysis how sensitive the spectmm line shape is to the molecular organization at the liquid/Hquid boundary. As discussed in Section 4.3.2, vibrational sum frequency generation studies of the neat water/DCE interface provide a rather different conclusion. 

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