posted on 2015-12-16, 20:54authored byVictor
B. Oyeyemi, John A. Keith, Michele Pavone, Emily A. Carter
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.