Assessment of double-hybrid energy functionals for pi-conjugated systems.

There have been tremendous efforts in the past decade on the use of computational methods for conjugated systems. Their properties and energetics are often described by density functional theory calculations which, however, are known to face a challenge when dealing with these systems since serious and systematic errors with popular density functionals occur, specially in the case of having stacked or sterically overcrowded aromatic systems, and discourage their use as a black box technique. We overcome here this shortcoming by applying recently developed dispersion-corrected double-hybrid density functionals (B2PLYP) in search of greater yet wide accuracy with little more computational effort. Interestingly, we have derived a related method (B2piPLYP), which has been thoroughly assessed against a set of databases and reactions of the most interest, and works better for this subclass of systems. The deviations with respect to benchmark or experimental values are found to be in the reasonably low range of 1-2 kcal/mol when a correction for the dispersion interactions is added and, most importantly, without suffering the large and systematic errors that are common in former yet conventional methods.