Jim Lattimer's involvement with high-performance computing centers on developing the microphysics that is a key ingredient in massively-parallel simulations of supernovae, proto-neutron stars, and neutron star mergers. This mainly centers on the equation of state and opacities of dense matter, with densities ranging from a few g/cc to 10 times the nuclear saturation density (i.e., 3x10^{15} g/cc), temperatures from 10^9 K to 10^{11} K, and average proton fractions from 0.5 to essentially 0 (i.e., pure neutron matter). In our present modeling, we approximate nuclei as compressible liquid drops immersed in a sea of uniform nuclear matter. The underlying nuclear forces are derived from both non-relativistic potential models and relativistic field-theoretical interactions. A future goal is to replace the liquid droplet ansatz with a three-dimensional Thomas-Fermi approach. Jim Lattimer's work is described in more detail on his homepage.