A less expensive Ewald lattice sum

We present a treatment of the Ewald lattice sum which permits 25% or more decrease in program execution cost for the same level of accuracy. This is accomplished by optimizing on additional degrees of freedom introduced into the function that partitions the Coulombic potential between real- and reciprocal-space parts. The technique was tested in simulations of 1 M KCl in water. It is relatively simple to implement in existing codes, including those based on fast-Fourier-transform solutions to the lattice sum.