Publication informationChemSusChem, 4 (4): 474-479Publisher WileyLink to online version http://dx

Nonequilibrium molecular dynamics simulations of water have been performed in the isothermal–isobaric ensemble in the presence of external electromagnetic fields of varying intensity in the microwave to far-infrared frequency range, using a rigid/polarizable and a flexible/nonpolarizable potential model, from 260 to 400 K. Significant alterations in molecular mobility and hydrogen bonding patterns were found vis-à-vis zero-field conditions. In addition, the influence of the isothermal–isobaric ensemble on these observations was gauged by means of comparison with pure Newtonian simulation findings in the presence of external fields, and the former results were in reasonable accord with the latter.

Nonequilibrium molecular dynamics simulations of water in an intense external microwave field have been performed using a rigid/polarizable and a flexible/nonpolarizable potential model, from ambient conditions to supercriticality. The heating profiles were compared to that predicted from a macroscopic energy balance, and the polarizable model was found to be superior in this regard.

Nonequilibrium molecular-dynamics (MD) simulations have been performed for the growth and dissolution of a spherical methane hydrate crystallite, surrounded by a saturated water-methane liquid phase, in both the absence and presence of external electromagnetic (e/m) fields in the microwave to far infrared range (5-7500 GHz) at root-mean square (rms) electric field intensities of up to 0.2 V/A. A rigid/polarizable potential was used to model water and a rigid/nonpolarizable model was utilized for methane. In the absence of a field, it was found that the average growth rate of the crystallite was approximately 0.32 water and 0.045 methane molecules per picosecond, evaluated over a 500 ps NPT simulation for three different initial geometries. Upon the application of an e/m field, it was found that no significant deviations from the zero-field crystal growth patterns were observed for rms electric field intensities of less than about 0.1 V/A, regardless of the field frequency. At, and above, this "threshold" intensity, it was found that dissolution took place. The mobility of the molecules in the system was enhanced by the e/m field, to the greatest extent for frequencies of 50-100 GHz. Furthermore, it was observed that there was a systematic frequency variation in the pattern of dipole alignment with the external field and this led to marked differences in the rate of dissolution.

Equilibrium molecular dynamics (MD) simulations have been performed in both the NVT and NPT ensembles to study the structural and dynamical properties of fully occupied methane clathrate hydrates at 50, 125, and 200 K. Five atomistic potential models were used for water, ranging from fully flexible to rigid polarizable and nonpolarizable. A flexible and a rigid model were utilized for methane. The phonon densities of states were evaluated and the localized rattling modes for the methane molecules were found to couple to the acoustic phonons of the host lattice. The calculated methane density of states was found to be in reasonable agreement with available experimental data.

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Water self-diffusion within human aquaporin 4 has been studied using molecular dynamics (MD) simulations in the absence and presence of external ac and dc electric fields. The computed diffusive (p(d)) and osmotic (p(f)) permeabilities under zero-field conditions are (0.718 ± 0.24) × 10(-14) cm(3) s(-1) and (2.94 ± 0.47) × 10(-14) cm(3) s(-1), respectively; our p(f) agrees with the experimental value of (1.50 ± 0.6) × 10(-14) cm(3) s(-1). A gating mechanism has been proposed in which side-chain dynamics of residue H201, located in the selectivity filter, play an essential role. In addition, for nonequilibrium MD in external fields, it was found that water dipole orientation within the constriction region of the channel is affected by electric fields (e-fields) and that this governs the permeability. It was also found that the rate of side-chain flipping motion of residue H201 is increased in the presence of e-fields, which influences water conductivity further.

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