  Which density functional is the "best" for structure simulations of a
particular material? A concise, first principles, approach to answer this
question is presented. The random phase approximation (RPA)--- an accurate many
body theory--- is used to evaluate various density functionals. To demonstrate
and verify the method, we apply it to the hybrid perovskite MAPbI$_3$, a
promising new solar cell material. The evaluation is done by first creating
finite temperature ensembles for small supercells using RPA molecular dynamics,
and then evaluating the variance between the RPA and various approximate
density functionals for these ensembles. We find that, contrary to recent
suggestions, van der Waals functionals do not improve the description of the
material, whereas hybrid functionals and the SCAN (strongly constrained
appropriately normed) density functional yield very good agreement with the
RPA. Finally, our study shows that in the room temperature tetragonal phase of
MAPbI$_3$, the molecules are preferentially parallel to the shorter lattice
vectors but reorientation on ps timescales is still possible.
