  Bulk viscosity of a plasma consisting of strongly coupled diatomic ions is
computed using molecular dynamics simulations. The simulations are based on the
rigid rotor one-component plasma, which is introduced as a model system that
adds two degrees of molecular rotation to the traditional one-component plasma.
It is characterized by two parameters: the Coulomb coupling parameter,
$\Gamma$, and the bond length parameter, $\Omega$. Results show that the
long-range nature of the Coulomb potential can lead to long rotational
relaxation times, which in turn yield large values for bulk viscosity. The
bulk-to-shear viscosity ratio is found to span from small to large values
depending on the values of $\Gamma$ and $\Omega$. Although bulk viscosity is
often neglected in plasma modeling, these results motivate that it can be large
in molecular plasmas with rotational degrees of freedom.
