Assessment
of Orbital-Optimized MP2.5 for Thermochemistry
and Kinetics: Dramatic Failures of Standard Perturbation Theory Approaches
for Aromatic Bond Dissociation Energies and Barrier Heights of Radical
Reactions
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Abstract
An assessment of orbital-optimized
MP2.5 (OMP2.5) [Bozkaya, U.; Sherrill, C. D. J. Chem.
Phys. 2014, 141, 204105] for thermochemistry and kinetics is presented. The OMP2.5
method is applied to closed- and open-shell reaction energies, barrier
heights, and aromatic bond dissociation energies. The performance
of OMP2.5 is compared with that of the MP2, OMP2, MP2.5, MP3, OMP3,
CCSD, and CCSD(T) methods. For most of the test sets, the OMP2.5 method
performs better than MP2.5 and CCSD, and provides accurate results.
For barrier heights of radical reactions and aromatic bond dissociation
energies OMP2.5–MP2.5, OMP2–MP2, and OMP3–MP3
differences become obvious. Especially, for aromatic bond dissociation
energies, standard perturbation theory (MP) approaches dramatically
fail, providing mean absolute errors (MAEs) of 22.5 (MP2), 17.7 (MP2.5),
and 12.8 (MP3) kcal mol<sup>–1</sup>, while the MAE values
of the orbital-optimized counterparts are 2.7, 2.4, and 2.4 kcal mol<sup>–1</sup>, respectively. Hence, there are 5–8-folds
reductions in errors when optimized orbitals are employed. Our results
demonstrate that standard MP approaches dramatically fail when the
reference wave function suffers from the spin-contamination problem.
On the other hand, the OMP2.5 method can reduce spin-contamination
in the unrestricted Hartree–Fock (UHF) initial guess orbitals.
For overall evaluation, we conclude that the OMP2.5 method is very
helpful not only for challenging open-shell systems and transition-states
but also for closed-shell molecules. Hence, one may prefer OMP2.5
over MP2.5 and CCSD as an <i>O</i>(<i>N</i><sup>6</sup>) method, where <i>N</i> is the number of basis
functions, for thermochemistry and kinetics. The cost of the OMP2.5
method is comparable with that of CCSD for energy computations. However,
for analytic gradient computations, the OMP2.5 method is only half
as expensive as CCSD