  Multimaterial heat diffusion can be a challenging numerical problem when the
material boundaries are misaligned with the numerical grid. Even when the
boundaries start out aligned, they typically become misaligned through
hydrodynamic motion. There are usually a number of methods for handling
multimaterial cells in any given hydro code. One of the simplest methods is to
replace the multimaterial cell by an average single-material cell whose heat
capacity and conductivity are averages over the constituent materials. One can
further refine this model by using either the arithmetic or harmonic averages,
thereby providing two distinct (albeit naive) multimaterial models for the
arithmetic and harmonic averages. More sophisticated models typically involve a
surrogate mesh of some kind, as with the thin mesh and static condensation
methods. In this paper, we perform rigorous code verification of the
multiphysics hydrocode FLAG, including grid resolution studies. We employ a
number of newly constructed 2D heat flow solutions that generalize the standard
{\em planar sandwich} solution, and this paper offers a smorgasbord of exact
solutions for heat flow verification. To perform the analyses and to produce
the corresponding convergence plots, we employ the code verification tool
ExactPack.
