Accurate Hydrogen Positions in Organic Crystals: Assessing a Quantum-Chemical Aide

Organic molecules crystallize in manifold structures. The last few decades have seen the rise of high-resolution X-ray diffraction techniques that make the structures of even the most complex crystals easily accessible. Still, an intrinsic challenge lies in assigning hydrogen atoms’ positions from X-ray experiments alone. Quantum chemistry plays a fruitful, complementary role here, and so ab initio optimization techniques for organic crystals are on the rise as well. In this context, we review and evaluate a popular ab initio strategy based on plane-wave density-functional computations, namely, selectively relaxing H positions in an otherwise fixed cell. Our data show that such-optimized C–H, N–H, O–H, and B–H bond lengths coincide well with results from neutron diffraction—the experimental technique that sets the “gold standard” for H positions in molecular crystals but which is far less easily available. We have thus justified the use of a quantum-chemical aide with a broad variety of possible applications.