The bulk viscosity of molecular models of gases and liquids is determined by molecular simulations as a
combination of a dilute gas contribution, arising due to the relaxation of internal degrees of freedom, and a
configurational contribution, due to the presence of intermolecular interactions. The dilute gas contribution
is evaluated using experimental data for the relaxation times of vibrational and rotational degrees of freedom.
The configurational part is calculated using Green-Kubo relations for the fluctuations of the pressure tensor
obtained from equilibrium microcanonical molecular dynamics simulations. As a benchmark, the Lennard-
Jones fluid is studied. Both atomistic and coarse-grained force fields for water, CO
2
and n-decane are
considered and tested for their accuracy. Comparison to experimental data, where present, demonstrates
that the tested models show various degrees of success in predicting bulk viscosity values, although atomistic
force fields in general seem to perform more consistently than the corresponding coarse-grained counterparts.
The dilute gas contribution to the bulk viscosity is seen to be significant only in the cases when intramolecular
relaxation times are in the
μ
s range, and for low vibrational wave numbers (
<
1000 cm
−
1
); This explains
the abnormally high values of bulk viscosity reported for CO
2
. In all other cases studied, the dilute gas
contribution is negligible, and the configurational contribution dominates the overall behaviour. In particular,
the configurational term is responsible for the enhancement of the bulk viscosity near the critical point.