Accuracy of TD-DFT Geometries: A Fresh Look

Abstract

We benchmark a panel of 48 DFT exchange-correlation functionals in the framework of TD-DFT optimizations of the geometry of valence singlet excited states. To this end, we use a set of 41 small- and medium-sized organic molecules for which reference geometries were obtained at high level of theory, typically, CC3 or CCSDR(3), with the aug-cc-pVTZ atomic basis set. For the ground-state parameters, the tested functionals provide average deviations that are small (0.010 Å and 0.5° for bond lengths and valence angles) and not very sensitive to the selected (hybrid) functional, but the errors are larger for the most polarized bonds (CO, CN, and so on). Nevertheless, DFT has a tendency to provide too compact distances, a trend slightly enhanced for functionals including a large amount of exact exchange. The average errors largely increase when going to the excited-state for most bond types, that is, TD-DFT delivers less accurate excited-state distances than DFT for ground state. In particular TD-DFT combined with hybrid functionals provides significantly too short CO and CS/CSe bonds with respective average errors in the -0.026/-0.052 Å and -0.015/-0.082 Å ranges, depending on the selected hybrid functional. For the carbonyl bonds, the sizes of the TD-DFT deviations obtained when selecting standard hybrid functionals are of the same order of magnitude as the EOM-CCSD ones.

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

“…The PBE0 functional was chosen based on benchmark papers for heterocyclic rings. [39][40] Stationary points with no imaginary frequencies were confirmed through frequency calculations. The stability of the wave function and spin contamination for the triplet state were checked.…”

“…Time‐dependent DFT (TD‐DFT) was used for the S 1 state optimizations while the unrestricted version of DFT was used for T 1 state optimizations. The PBE0 functional was chosen based on benchmark papers for heterocyclic rings . Stationary points with no imaginary frequencies were confirmed through frequency calculations.…”

How accurate are TD‐DFT excited‐state geometries compared to DFT ground‐state geometries?

“…More recently, Brémond et al [ 65 ] reported the first extensive assessment of the accuracy of TD‐DFT for geometries of small organic molecules in valence excited states. Testing a panel of 48 different functionals and using geometries previously calculated [ 66 ] with CASPT2 or a high‐order coupled cluster (CC) method (CC3 [ 67 ] or CCSDR(3) [ 68 ] ) as reference, these authors concluded that excited‐state bonds lengths obtained with TD‐DFT are on average of somewhat lesser quality than the corresponding DFT ground‐state bond lengths.…”

Computation of covalent and noncovalent structural parameters at low computational cost: Efficiency of the DH‐SVPD method

“…Furthermore, we want to verify that the observed improvement on energy properties of weakly interacting molecules is not performed at the expense of a basic property such as molecular geometry but that the performance improvement on both properties stays in close correlation. [21,37] Our benchmarking investigation focuses on the CCse and B3se databases of accurate structural parameters introduced some years ago by Barone and coworkers. [38][39][40][41] The former includes a collection of 21 small organic molecules whose reference covalent structural parameters are derived from a mixed coupled-cluster singles and doubles plus perturbative triples [CCSD(T)] and experimental protocol.…”