Modified Opposite-Spin-Scaled Double-Hybrid Functionals.

We investigate the potential performance improvements of double-hybrid density functionals by replacing the standard scaled opposite-spin MP2 (SOS-MP2) with the modified opposite-spin-scaled MP2 (MOS-MP2) in the nonlocal correlation component. Using the large and diverse GMTKN55 data set, we find that MOS-double hybrids provide significantly better accuracy compared to SOS-MP2-based double hybrids when empirical dispersion correction is not employed. The noncovalent interaction subsets account for the majority of this improvement. However, when the D4 dispersion correction is applied, the performance gap between MOS-MP2- and SOS-MP2-based double hybrids becomes negligible. While the new methods do not outperform the current state-of-the-art double hybrid functionals, our study offers valuable insights into the applicability of distance-dependent MP2 in place of conventional SOS-MP2, as well as the critical role of empirical dispersion corrections in further enhancing accuracy─insights that are useful for guiding future method developments. For nine transition metal sets, dispersion-corrected spin-component-scaled double hybrids are still significantly better than any MOS-double hybrid functional.