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Abstract:
Aspirin has been used and studied for over a century, but has only recently been shown to have
an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin
are degenerate in terms of lattice energy, kinetic eects have been suggested to determine the
metastability of the less abundant form II. Here, rst-principles calculations provide an alternative
explanation based on free-energy dierences at room temperature. The explicit consideration of
many-body van der Waals interactions in the free energy demonstrates that the stability of the
most abundant form of aspirin is due to a subtle coupling between collective electronic
uctuations
and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the
two forms of aspirin rules out mechanical instability of form II as making it metastable
