Triplet-Tuning: A Novel Non-Empirical Construction Scheme of Exchange Functionals

Zhou Lin and Troy Van Voorhis (2018)

Highlighted by Jan Jensen

The absolute error of the optical band gap computed with the CC-PVDZ basis set

The difference between the lowest triplet and singlet energy (E_T) can be calculated both by UDFT and TDDFT but give different results because we don't know the exact density functional. So the authors suggest that functional can be improved by minimising this difference, i.e. without comparison to experimental data. 

Indeed, such a triplet tuned (TT) functional "provide more accurate predictions for key observables in photochemical measurements, including but not limited to E_T, optical band gaps (E_S), singlet–triplet gaps (∆E_ST), and ionization potentials (I)" for a set of 100 organic molecules. Two parameters in the PBE exchange functional, the fraction of short-range HF exchange and the range-separation parameter, were adjusted.

One thing that is not clear to me is if the equivalence of UDFT and TDDFT is valid only for the exact density. If so, this would only be valid in the complete basis set limit. Either way, the results clearly improve using the CC-PVDZ basis set.


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Intramolecular London Dispersion Interaction Effects on Gas-Phase and Solid-State Structures of Diamondoid Dimers

Fokin, A. A.; Zhuk, T. S.; Blomeyer, S.; Pérez, C.; Chernish, L. V.; Pashenko, A. E.; Antony, J.; Vishnevskiy, Y. V.; Berger, R. J. F.; Grimme, S.; Logemann, C.; Schnell, M.; Mitzel, N. W.; Schreiner, P. R., J. Am. Chem. Soc. 2017, 139, 16696-16707
Contributed by Steven Bacharach
Reposted from Computational Organic Chemistry with permission

Schreiner and Grimme have examined a few compounds (see these previous posts) with long C-C bonds that are found in congested systems where dispersion greatly aids in stabilizing the stretched bond. Their new paper¹ continues this theme by examining 1 (again) and 2, using computations, and x-ray crystallography and gas-phase rotational spectroscopy and electron diffraction to establish the long C-C bond.

The distance of the long central bond in 1 is 1.647 Å (x-ray) and 1.630 Å (electron diffraction). Similarly, this distance in 2 is 1.642 Å (x-ray) and 1.632 Å (ED). These experiments discount any role for crystal packing forces in leading to the long bond.

A very nice result from the computations is that most functionals that include some dispersion correction predict the C-C distance in the optimized structures with an error of no more than 0.01 Å. (PW6B95-D3/DEF2-QZVP structures are shown in Figure 1.) Not surprisingly, HF and B3LYP without a dispersion correction predict a bond that is too long.) MP2 predicts a distance that is too short, but SCS-MP2 does a very good job.

1

2

Figure 1. PW6B95-D3/DEF2-QZVP optimized structures of 1 and 2.

References

1) Fokin, A. A.; Zhuk, T. S.; Blomeyer, S.; Pérez, C.; Chernish, L. V.; Pashenko, A. E.; Antony, J.; Vishnevskiy, Y. V.; Berger, R. J. F.; Grimme, S.; Logemann, C.; Schnell, M.; Mitzel, N. W.; Schreiner, P. R., "Intramolecular London Dispersion Interaction Effects on Gas-Phase and Solid-State Structures of Diamondoid Dimers." J. Am. Chem. Soc. 2017, 139, 16696-16707, DOI: 10.1021/jacs.7b07884.

InChIs

1: InChI=1S/C28H38/c1-13-7-23-19-3-15-4-20(17(1)19)24(8-13)27(23,11-15)28-12-16-5-21-18-2-14(9-25(21)28)10-26(28)22(18)6-16/h13-26H,1-12H2
InChIKey=MMYAZLNWLGPULP-UHFFFAOYSA-N

2: InChI=1S/C26H34O2/c1-11-3-19-15-7-13-9-25(19,21(5-11)23(27-13)17(1)15)26-10-14-8-16-18-2-12(4-20(16)26)6-22(26)24(18)28-14/h11-24H,1-10H2
InChIKey=VPBJYHMTINJMAE-UHFFFAOYSA-N

'
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