We report a study of viscosity by the method of Helfand moment in systems with periodic boundary conditions. We propose a new definition of Helfand moment which takes into account the minimum image convention used in molecular dynamics with periodic boundary conditions. Our Helfand-moment method is equivalent to the method based on the Green-Kubo formula and is not affected by ambiguities due to the periodic boundary conditions. Moreover, in hard-ball systems, our method is equivalent to that developed by Alder, Gass, and Wainwright [J. Chem. Phys. 53, 3813 (1970)]. We apply and verify our method in a fluid composed of $N>~2$ hard disks in elastic collisions. We show that the viscosity coefficients already take values in good agreement with Enskog’s theory for $N=2\mathrm{}$ hard disks in a hexagonal geometry.

• Received 7 February 2003

DOI:https://doi.org/10.1103/PhysRevE.68.041204