A hybrid method is introduced and applied that combines second-order Moller Plesset perturbation theory (MP2) for cluster models with density functional theory for periodic models to obtain structures and energetics for molecular crystals of the alkane species, notably propane. Calculated lattice energies elucidate the fact that propane has the lowest melting temperature of all organic compounds. The hybrid method provides an alternative to currently existing methods, which use either periodic MP2 for the determination of periodic crystals or periodic DFT+D. We also compare the hybrid method to a number of modern density functionals which are commonly used for the computation of molecular crystals. Depending on the functional used, we get a large range of lattice energies. From our QM:QM results, we can deduce that the achievements of DFT+D for computing molecular crystals largely relies on error cancellation effects between the functional and its dispersion coefficients.
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