  Molecular crystals play a central role in a wide range of scientific fields,
including pharmaceuticals and organic semiconductor devices. However, they are
challenging systems to model accurately with computational approaches because
of a delicate interplay of intermolecular interactions such as hydrogen bonding
and van der Waals dispersion forces. Here, by exploiting recent algorithmic
developments, we report the first set of diffusion Monte Carlo lattice energies
for all 23 molecular crystals in the popular and widely used X23 dataset.
Comparisons with previous state-of-the-art lattice energy predictions (on a
subset of the dataset) and a careful analysis of experimental sublimation
enthalpies reveals that high-accuracy computational methods are now at least as
reliable as (computationally derived) experiments for the lattice energies of
molecular crystals. Overall, this work demonstrates the feasibility of
high-level explicitly correlated electronic structure methods for broad
benchmarking studies in complex condensed phase systems, and signposts a route
towards closer agreement between experiment and simulation.
