  Extensive research has been carried out in the past to estimate the bulk
viscosity of dense monatomic fluids; however, little attention has been paid to
estimate the same in the dilute gas regime. In this work, we perform precise
Green-Kubo calculations in molecular dynamics simulations to estimate the bulk
viscosity of dilute argon gas. The investigated temperature and pressure range
is 300 to 750 K and 0.5 to 1.5 bar respectively. It is observed that the
estimated bulk viscosity is $O(10^{-10})$ Pa s, which is several orders of
magnitude smaller than that of other diatomic and polyatomic gases, but
nonetheless, not an absolute zero as typically assumed. It implies that Stokes'
hypothesis is true for dilute monatomic gases only in an approximate rather
than absolute sense. The variation of bulk viscosity with pressures at constant
temperatures has also been studied and is found to be of quadratic nature. The
obtained bulk viscosity values are also reported in the reduced Lennard-Jones
units to enable extension of the present results to other noble gases as well.
It has also been observed that the bulk viscosity of Lennard-Jones monatomic
gases becomes temperature independent at very low densities.
