XYG3s: Speedup of the XYG3 fifth-rung density functional with scaling-all-correlation method

Recently, we proposed a new version of doubly hybrid functional, XYG3 [Y. Zhang, X. Xu, and W. A. Goddard III, Proc. Natl. Acad. Sci. U.S.A. 106, 4963 (2009)], which not only has a nonlocal orbital-dependent component in the exchange term (Hartree-Fock-like exchange), but also contains information about the unoccupied Kohn-Sham orbitals in the correlation part (PT2 double excitation). In the present work, we examine the frozen-core approximation and Truhlar's scaling-all-correlation (SAC) method in order to accelerate the PT2 evaluations. We adopt the SAC idea and extend XYG3 to propose XYG3s that uses a uniform scaling parameter for the PT2 correlation. We conclude that XYG3s is both fast and accurate for thermochemistry, bond dissociation enthalpies, reaction barrier heights, and nonbonded interactions of main group molecules. In addition, the accuracy remains nearly constant with system size.