Do Optimally Tuned Range-Separated Hybrid Functionals Require a Reparametrization of the Dispersion Correction? It Depends

For ground- and excited-state studies of large molecules, it is the state of the art to combine (time-dependent) DFT with dispersion-corrected range-separated hybrid functionals (RSHs), which ensures an asymptotically correct description of exchange effects and London dispersion. Specifically for studying excited states, it is common practice to tune the range-separation parameter omega (optimal tuning), which can further improve the accuracy. However, since optimal tuning essentially changes the functional, it is unclear if and how much the parameters used for the dispersion correction depend on the chosen omega value. To answer this question, we explore this interdependency by refitting the DFT-D4 dispersion model for six established RSHs over a wide range of omega values (0.05-0.45 a (0) (-1)) using a set of noncovalently bound molecular complexes. The results reveal some surprising differences among the investigated functionals: While PBE-based RSHs and omega B97M-D4 generally exhibit a weak interdependency and robust performance over a wide range of omega values, B88-based RSHs, specifically LC-BLYP, are strongly affected. For these, even a minor reduction of omega from the default value manifests in strong systematic overbinding and poor performance in the typical range of optimally tuned omega values. Finally, we discuss strategies to mitigate these issues and reflect the results in the context of the employed D4 parameter optimization algorithm and fit set, outlining strategies for future improvements.

Automated summary

This study investigates the interdependency between dispersion correction parameters and the range-separation parameter omega in large molecules. The results show that some functionals are strongly affected by omega values, leading to overbinding and poor performance. Strategies to mitigate these issues are discussed, providing insights for future improvements.