Insufficient Hartree–Fock Exchange in Hybrid DFT Functionals Produces Bent Alkynyl Radical Structures

Abstract

Density functional theory (DFT) is often used to determine the electronic and geometric structures of molecules. While studying alkynyl radicals, we discovered that DFT exchange-correlation (XC) functionals containing less than ∼22% Hartree-Fock (HF) exchange led to qualitatively different structures than those predicted from ab initio HF and post-HF calculations or DFT XCs containing 25% or more HF exchange. We attribute this discrepancy to rehybridization at the radical center due to electron delocalization across the triple bonds of the alkynyl groups, which itself is an artifact of self-interaction and delocalization errors. Inclusion of sufficient exact exchange reduces these errors and suppresses this erroneous delocalization; we find that a threshold amount is needed for accurate structure determinations. Below this threshold, significant errors in predicted alkyne thermochemistry emerge as a consequence.

The effect of the Perdew-Zunger self-interaction correction to density functionals on the energetics of small molecules

“…For all angles, bond lengths have been optimized. For PBE the energy drops slightly when the molecule starts to bend, with an optimal angle in good agreement with the previous study 52 . For PBE+SIC and PBE+SIC/2, the linear geometry is favored.…”

“…The bond lengths have been optimized for each value of the bond angle. The PBE ground state geometry is bent with an angle of ≈166 • in agreement with previous calculations52 . PBE+SIC and PBE+SIC/2 both favor the linear geometry in agreement with experiment50 .…”

Evaluating the electronic structure of formal LnIIions in LnII(C5H4SiMe3)31−using XANES spectroscopy and DFT calculations

“…1b , 1c , 8a , 8d That is to say, GGA and hybrid functionals with lower HF exchange percentages fail to give the correct Ho II (C 5 H 4 SiMe 3 ) 3 1– spin state, which is likely attributable to the delocalization error. 20 , 21 Many reports have described how increasing HF exchange improves the calculated energetics by DFT-based methods such as excitation energy, 22 thermochemical kinetics, 23 reaction barriers, 24 and electron detachment energy. 25 Consistently, our DFT/PBE calculated Ho III (4f 10 5d 0 )–C centroid and Ho II (4f 10 5d 1 )–C centroid distances are in excellent agreement with experimental values ( Table 2 ), while the Ho II (4f 11 5d 0 )–C centroid distances are longer than the experimental results by ∼0.1 Å.…”

Surface chemistry of oxygen on aluminum-Performance of the density functionals: PBE, PBE0, M06, and M06-L

“…This has been demonstrated to happen when pure DFT was employed to model radical species, semiconductors, and insulators . Additionally the nonlocal component of the energy of some open‐shell species are correctly described only with functionals that incorporate nonlocal exchange such as the hybrid HF‐DFT functionals . In some of these cases not even geometries are correctly described with pure DFT.…”

“…[33][34][35] Additionally the nonlocal component of the energy of some open-shell species are correctly described only with functionals that incorporate nonlocal exchange such as the hybrid HF-DFT functionals. [36] In some of these cases not even geometries are correctly described with pure DFT. In contrast, in general, hybrid HF-DFT has poor performance for describing some transition metal complexes and solid metals when compared with the nonhybrid functionals.…”