Tight binding molecular dynamics studies of the viscosity of liquid selenium

A tight binding technique is employed to study the influence of the size of the unit cell on molecular dynamics simulations of liquid selenium at 870 K. Systems with 69, 138, 276 and 552 atoms have been studied within the Fermi operator expansion method. The Green-Kubo formula has been used to calculate the shear viscosity for all systems from the off-diagonal component of the stress tensor. No dependence on the cell size could be found within the error bars defined by statistical accuracy. For the two smallest systems calculations up to 60 ps have been performed where the viscosities become identical. These findings indicate that the recently reported disagreement of DFT calculations and experiments on the viscosity of liquid selenium at 870 K must be attributed to a deficiency in the chosen exchange correlation functional rather than size effects.