Insufficient Hartree-Fock Exchange in Hybrid DFT Functionals Produces Bent Alkynyl Radical Structures.
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.
Departments of †Chemical and Biological Engineering and ‡Mechanical and Aerospace Engineering, §Program in Applied and Computational Mathematics, and ∥Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey, 08544-5263, United States.
Victor B
VB
Oyeyemi
Departments of †Chemical and Biological Engineering and ‡Mechanical and Aerospace Engineering, §Program in Applied and Computational Mathematics, and ∥Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey, 08544-5263, United States.
John A
JA
Keith
Departments of †Chemical and Biological Engineering and ‡Mechanical and Aerospace Engineering, §Program in Applied and Computational Mathematics, and ∥Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey, 08544-5263, United States.
Michele
M
Pavone
Departments of †Chemical and Biological Engineering and ‡Mechanical and Aerospace Engineering, §Program in Applied and Computational Mathematics, and ∥Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey, 08544-5263, United States.
Emily A
EA
Carter
ab initio calculations; alkynyl, alkylethynyl radicals; delocalization error; density functional theory; exchange-correlation functionals; self-interaction error; thermochemistry