for RI-MP2).
Thus increasing basis set size for these density functional methods, at least from double zeta to triple zeta, does improve accuracy, albeit at a significant computational cost. In general, the B97-3c method provides accuracy comparable to popular dispersion-corrected DFT methods such as B3LYP-D3BJ with faster performance, and RI-MP2 provides greater accuracy at a very similar speed.

Dispersion Corrections

Since the bonding is consistent across multiple conformers, the ranking of small energy differences is known to be dominated by non-bonding interactions.\cite{Martin_2013,Gruzman_2009} Density functional methods are known to incorrectly account for dispersion interactions, which has led to a variety of empirical corrections.\cite{Grimme_2011,Becke_2005,Johnson_2005,Johnson_2006,Caldeweyher_2017,Caldeweyher_2019,Grimme_2011a,Johnson_2009} Comparing un-corrected PBE, B3LYP, and ωB97X single-point energies to DLPNO-CCSD(T) illustrate a significant effect. The uncorrected median R2 values drop by ~0.12, and the median Spearman correlations drop by ~0.08. For example, the median R2 of B3LYP / TZ drops from 0.920 to 0.706 without the D3BJ dispersion correction.