Water clusters energetics

Among the recently published works, the one which showed that the cyclic structures of water clusters open up to form a linear structure above a certain threshold electric field value a was a systematic ab initio study on the effect of electric field on structure, energetics, and transition states of trimer, tetramer, and pentamer water clusters (both cyclic and acyclic) [36], Considering c/.v-butadiene as a model system, the strength and the direction of a static electric field has been used to examine the delocalization energy, the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations [37]. Another recent work performed by Rai et al. focused on the studies using the DFT and its time-dependent counterpart of effects of uniform static electric field on aromatic and aliphatic hydrocarbons [38],... 

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... 

The proton transfer in ionized phenol-water clusters is strongly dependent on the number of water molecules and their specific organization, i.e., the PT is a process assisted by the solvent [72], Most of the theoretical studies of PT in [C6H50H-(H20) ],+ clusters were focused on the structure, vibrational, [79,80,81] and energetic aspects [77,78]. However, much less is known on the dynamics of PT. 

Xantheas and co-workers [159,160] have incorporated polarization in a model scheme and have used that to provide a clear basis for the enhancement of water s dipole in ice. A model potential with polarization has been reported for the formaldehyde dimer [161]. It is an example of a carefully crafted potential, which is system-specific because of its application to pure liquid formaldehyde, but which has terms associated with properties and interaction elements as in the above models. As well, some of the earliest rigid-body DQMC work, which was by Sandler et al. [162] on the nitrogen-water cluster, used a potential expressed in terms of interaction elements derived from ab initio calculations with adjustment (morphing). Stone and co-workers have developed interaction potentials for HF clusters [163], water [164], and the CO dimer [165], which involve monomer electrical properties and terms derived from intermolecular perturbation theory treatment. SAPT has been used for constructing potentials that have enabled simulations of molecules in supercritical carbon dioxide [166]. There are, therefore, quite a number of models being put forth wherein electrical analysis and/or properties of the constituents play an essential role, and some where electrical analysis is used to understand property changes as well as the interaction energetics. 

A water molecule favors four H-bond interactions. Neutral water clusters are determined mainly by the H-bond interactions and strains between water molecules. Structures, energetics, and electronic and spectroscopic properties of small water clusters have been well investigated [13-15]. Each water molecule in... 

Figure 1. The lowest-energy neutral water cluster structures (W2 i2 at 0 K) and four other iso-energetic isomers of neutral water hexamer.

The MP2 method has been found to accurately describe the energetics of water clusters, providing that sufficiently flexible atomic basis sets are employed [5,11,20, 67,68]. However, whUe MP2 calculations are feasible for clusters containing up to 30 or so water molecules, the steep 0 N )) computational scaling of conventional MP2 calculations with system size precludes their use in carrying out Monte Carlo or molecular dynamics simulations of water clusters containing six or more monomers. 

S.S. Xantheas, C.J. Burnham, and R.J. Harrison, Development of transferable interaction models for water. II. Accurate energetics of the first few water clusters from first principles, J. Chem. Phys. 116 (2002), pp. 1493-1499. 

Free energetics of Nal contact and solvent-separated ion pairs in water clusters ... 

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