**Non-equilibrium simulations of thermally induced electric fields in
water**

P Wirnsberger and D Fijan and A Saric and M Neumann and C Dellago and D Frenkel, JOURNAL OF CHEMICAL PHYSICS, 144, 224102 (2016).

DOI: 10.1063/1.4953036

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 x 10(7) and 5 x 10(7) V/m for a temperature gradient
of 5.2 K/angstrom. Our value for the Wolf method is therefore an order
of magnitude lower than the literature value **J. A. Armstrong and F.
Bresme, J. Chem. Phys. 139, 014504 (2013); J. Armstrong et al., J. Chem.
Phys. 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. Published by AIP Publishing.

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