Density functional theory is becoming the method of choice for organic molecular crystal structure prediction thanks to many successful predictions.  While standard dispersion-corrected generalized gradient approximation and hybrid functionals perform well in many molecular crystal systems, there are also clear cases where these models fail due to problems such as delocalization error and difficulties in describing non-covalent interactions.  We will discuss these behaviors through a number of examples in conformational polymorphs, which are crystals where different intramolecular molecular conformations enables different intermolecular crystal packings.  These systems will be approached from both the bulk perspective, using periodic DFT, and from the fragment perspective, using various density functionals and correlated wavefunction models to examine 1-body and 2-body interactions in the crystals.  Key findings include (1) substantial systematic errors in systems where conformational changes alter the extent of pi-conjugation/delocalization and (2) the complex role of error cancellation among fragment contributions and the overall successes of periodic DFT for organic crystals.  Strategies for addressing these limitations will also be discussed.

How the pieces add up: Investigating the performance of common DFT functionals in organic crystals

ACS Spring 2022

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