title: Double hybrid functionals and the Π-system bond length alternation challenge: rivaling accuracy of post-HF methods.
abstract: Predicting accurate bond length alternations (BLAs) in long conjugated oligomers has been a significant challenge for electronic-structure methods for many decades, made particularly important by the close relationships between BLA and the rich optoelectronic properties of π-delocalized systems. Here, we test the accuracy of recently developed, and increasingly popular, double hybrid (DH) functionals, positioned at the top of Jacobs Ladder of DFT methods of increasing sophistication, computational cost, and accuracy, due to incorporation of MP2 correlation energy. Our test systems comprise oligomeric series of polyacetylene, polymethineimine, and polysilaacetylene up to six units long. MP2 calculations reveal a pronounced shift in BLAs between the 6-31G(d) basis set used in many studies of BLA to date and the larger cc-pVTZ basis set, but only modest shifts between cc-pVTZ and aug-cc-pVQZ results. We hence perform new reference CCSD(T)/cc-pVTZ calculations for all three series of oligomers against which we assess the performance of several families of DH functionals based on BLYP, PBE, and TPSS, along with lower-rung relatives including global- and range-separated hybrids. Our results show that DH functionals systematically improve the accuracy of BLAs relative to single hybrid functionals. xDH-PBE0 (N(4) scaling using SOS-MP2) emerges as a DH functional rivaling the BLA accuracy of SCS-MP2 (N(5) scaling), which was found to offer the best compromise between computational cost and accuracy the last time the BLA accuracy of DFT- and wave function-based methods was systematically investigated. Interestingly, xDH-PBE0 (XYG3), which differs to other DHs in that its MP2 term uses PBE0 (B3LYP) orbitals that are not self-consistent with the DH functional, is an outlier of trends of decreasing average BLA errors with increasing fractions of MP2 correlation and HF exchange.
authors.affiliation: Madrid Institute for Advanced Studies , IMDEA Nanoscience, Calle Faraday 9, Campus Cantoblanco, 28049 Madrid, Spain.
authors.forename: Michael
authors.initials: M
authors.lastname: Wykes
authors.affiliation: Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Department of Chemistry, Fudan University , Shanghai 200433, China.
authors.forename: Neil Qiang
authors.initials: NQ
authors.lastname: Su
authors.affiliation: Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Department of Chemistry, Fudan University , Shanghai 200433, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Xu
authors.affiliation: Institut de Recherche de Chimie Paris , IRCP CNRS UMR-8247, École Nationale Superieure de Chimie de Paris, Chimie ParisTech, 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France.|Institut Universitaire de France , 103 Boulevard Saint Michel, F-75005 Paris, France.
authors.forename: Carlo
authors.initials: C
authors.lastname: Adamo
authors.affiliation: Departamento de Química Física, Universidad de Alicante , E-03080 Alicante, Spain.
authors.forename: Juan-Carlos
authors.initials: JC
authors.lastname: Sancho-García