The manner in which electrolyte solutions respond to electric fields is
crucial to understanding the behavior of these systems both at, and away from,
equilibrium. The present formulation of linear response theory for such systems
is inconsistent with common molecular dynamics (MD) implementations. Using the
finite field formalism, suitably adapted for finite temperature MD, we
investigate the response of bulk aqueous NaCl solutions to both finite Maxwell
(E) and electric displacement (D) fields. The constant
E Hamiltonian allows us to derive the linear response relation for
the ionic conductivity in a simple manner that is consistent with the forces
used in conventional MD simulations. Simulations of a simple point charge model
of an electrolyte solution at constant E yield conductivities at
infinite dilution within 15% of experimental values. The finite field approach
also allows us to measure the solvent's dielectric constant from its
polarization response, which is seen to decrease with increasing ionic
strength. Comparison of the dielectric constant measured from polarization
response versus polarization fluctuations enables direct evaluation of the
dynamic contribution to this dielectric decrement, which we find to be small
but not insignificant. Using the constant D formulation, we also
rederive the Stillinger-Lovett conditions, which place strict constraints on
the coupling between solvent and ionic polarization fluctuations.We are grateful for computational support from the UK Materials and Molecular Modelling Hub, which is partially funded by EPSRC (Grant No. EP/P020194), for which access was obtained via the UKCP consortium and funded by EPSRC Grant Ref. No. EP/P022561/1. S.J.C. is supported by a Royal Commission for the Exhibition of 1851 Research Fellowship