Hirshfeld surfaces identify inadequacies in computations of intermolecular interactions in crystals: Pentamorphic 1,8-dihydroxyanthraquinone

Fingerprint plots of Hirshfeld surfaces were used to locate and analyze the deficiencies in various methodologies employed in the determination of the relative energies of five polymorphs of 1,8-dihydroxyanthraquinone. Nine crystallographically independent molecules were characterized by X-ray crystallography and Hirshfeld surfaces were derived from the X-ray structures. The space groups and number_of independent molecules (Z′) for each of the polymorphs (1-5) is as follows: (1) P4₁ (or P4₃), Z′ = 1; (2) Pca2₁, Z′ = 2; (3) P1̄, Z = 4; (4) P2 ₁In, Z′ = 1; (5) P4₁2₁2 (or P4 ₃2₁2), Z′ = 0.5. Form 1 is the most thermodynamically stable among the reproducible structures, as established by competitive solubility tests, followed by 2 and then 4. The unrestrained structures of the five polymorphs were computed using the CVFF and COMPASS force fields as well as with the density functional code, SIESTA.