A Benchmark Quantum Monte Carlo Study of Molecular Crystal Polymorphism: A Challenging Case for Density-Functional Theory

Abstract

We 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.