Correct virial formulation in the isotropic periodic sum method.

The original formulation of the virial in the isotropic periodic sum (IPS) method assumes that the sphere defining the local region has a constant radius (the cutoff) independent of the system size. This assumption neglects a virial term originating from the separation between the local sphere and its periodic images. When comparing the IPS virial with that calculated from the cutoff plus long range correction method, the difference observed can be erroneously attributed to the representation of the infinite region. We show that when the two virials are calculated consistently the observed difference is significantly reduced. Additionally, the correct virial that includes the previously missing term is much simpler to calculate. We prove that in the IPS method the virial can be obtained as n/3 times the potential energy for the case of 1/r(n) type potentials.