Accuracy and reproducibility in crystal structure prediction: the curious case of ROY
We present the results of a new crystal structure prediction of ROY performed with the GRACE program, including the first wide search for Z′ = 2 structures. In addition, we present calculations of lattice-vibrational contributions to the polymorphs' relative stability. Because of excessive electron delocalization, the polymorphs of ROY constitute a surprisingly challenging system for density functional approximations, raising troubling questions regarding the performance of some of the most popular computational methods. Despite this, lattice dynamics calculations performed very well in reproducing the experimental cyano-group stretching frequencies, allowing us to propose a plausible candidate structure for the RPL polymorph, identified by comparing experimental Raman frequencies with calculated phonon modes.