Non-equilibrium simulations of thermally induced electric fields in water

Abstract

Using non-equilibrium molecular dynamics simulations, it has been recently demonstrated that water molecules align in response to an imposed temperature gradient, resulting in an effective electric field. Here, we investigate how thermally induced fields depend on the underlying treatment of long-ranged interactions. For the short-ranged Wolf method and Ewald summation, we find the peak strength of the field to range between $2\times 10^7$ and $5\times 10^7\text{V/m}$ for a temperature gradient of $5.2~\text{K/}\unicode{x212B}$. Our value for the Wolf method is therefore an order of magnitude lower than the literature value [J. Chem. Phys. 139, 014504 (2013) and 143, 036101 (2015)]. We show that this discrepancy can be traced back to the use of an incorrect kernel in the calculation of the electrostatic field. More seriously, we find that the Wolf method fails to predict correct molecular orientations, resulting in dipole densities with opposite sign to those computed using Ewald summation. By considering two different multipole expansions, we show that, for inhomogeneous polarisations, the quadrupole contribution can be significant and even outweigh the dipole contribution to the field. Finally, we propose a more accurate way of calculating the electrostatic potential and the field. In particular, we show that averaging the microscopic field analytically to obtain the macroscopic Maxwell field reduces the error bars by up to an order of magnitude. As a consequence, the simulation times required to reach a given statistical accuracy decrease by up to two orders of magnitude.

17 pages, 11 figures, link to supplementary material

References in corpus (6)

• An enhanced version of the heat exchange algorithm with excellent energy conservation properties
• Local molecular field theory for effective attractions between like charged objects in systems with strong Coulomb interactions
• Computing the dielectric constant of liquid water at constant dielectric displacement
• Interplay of local hydrogen-bonding and long-ranged dipolar forces in simulations of confined water
• Local molecular field theory for the treatment of electrostatics
• Damped reaction field method and the accelerated convergence of the real space Ewald summation

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• Generalized Moment Correction for Long-Ranged Electrostatics
• Real space pairwise electrostatic summation in a uniform neutralising background
• Seebeck coefficient of liquid water from equilibrium molecular dynamics
• Microscopic analysis of thermo-orientation in systems of off-centre Lennard-Jones particles