  Molecular dynamics (MD) simulations of complex electrochemical systems, such
as ionic liquid supercapacitors, are increasingly including the constant
potential method (CPM) to model conductive electrodes at specified potential
difference, but the inclusion of CPM can be computationally expensive. We
demonstrate the computational savings available in CPM MD simulations of ionic
liquid supercapacitors when the usual non-periodic slab geometry is replaced
with fully periodic boundary conditions. We show how a doubled cell approach,
previously used in non-CPM MD simulations of charged interfaces, can be used to
enable fully periodic CPM MD simulations. Using either a doubled cell approach,
or a finite field approach previously reported by others, fully periodic CPM MD
simulations produce comparable results to the traditional slab geometry
simulations with a nearly double speed-up in computational time. Indeed, these
savings can offset the additional cost of the CPM algorithm, resulting in
periodic CPM MD simulations that are faster than the non-periodic, fixed-charge
equivalent simulations for the ionic liquid supercapacitors studied here.
