Watson, Mark A.Hongo, KentaIitaka, ToshiakiAspuru-Guzik, Alan2012-12-172012Watson, Mark A., Kenta Hongo, Toshiaki Iitaka, and Alan Aspuru-Guzik. 2012. A Benchmark Quantum Monte Carlo Study of Molecular Crystal Polymorphism: A Challenging Case for Density-Functional Theory. In Advances in Quantum Monte Carlo, ed. Shigenori Tanaka, Stuart M. Rothstein, and William A. Lester, 1094:101-117. Washington, DC: American Chemical Society.97808412275071948-7185http://nrs.harvard.edu/urn-3:HUL.InstRepos:10057424We have applied the diffusion Monte Carlo (DMC) method, for the first time, to an organic molecular crystal (para-diiodobenzene) in order to determine the relative stability of its two well- known polymorphs. The DMC result predicts that the \(\alpha\) phase is more stable than the \(\beta\) phase at zero temperature, in agreement with experiment. In comparison, we evaluated four commonly-used local, semi-local and hybrid density functionals. We conclude that while density-functional theory (DFT) may provide the most practical method for including the effects of electron correlation, conventional functionals cannot be considered reliable for determining highly accurate energies in such systems.en-USMonograph or Book10.1021/bk-2012-1094.ch009