Phys. Rev. Lett. 113, 055701 (2014)

Acetylsalicylic acid, or aspirin, as it is more commonly called, has been known since ancient times for its analgesic properties. Over the past decade, the solid form used in the pharmaceutical product marketed worldwide has been shown to be only one of two possible polymorphs. The additional polymorph, known as form II, was predicted to be energetically very similar to form I, and the rather subtle difference between their structures required very careful structural studies for them to be eventually identified and distinguished experimentally. The question that naturally arose, therefore, was why form II was so much less abundant, and indeed seems to be less thermodynamically stable. Kinetic effects were examined first, and these may indeed play some role. But as Anthony Reilly and Alexandre Tkatchenko now show by means of first-principles calculations, long-range van der Waals interactions seem to be the key to understanding the two different behaviours. By explicitly including these many-body effects in their simulations, clear differences in the vibrational modes of the aspirin molecules are seen, uncovering a mechanism behind the stability of form I aspirin.