What Can We Learn about Dispersion from the Conformer Surface of <i>n</i>‑Pentane?

In earlier work [Gruzman, D.; Karton, A.; Martin, J. M. L. J. Phys. Chem. A 2009, 113, 11974], we showed that conformer energies in alkanes (and other systems) are highly dispersion-driven and that uncorrected DFT functionals fail badly at reproducing them, while simple empirical dispersion corrections tend to overcorrect. To gain greater insight into the nature of the phenomenon, we have mapped the torsional surface of <i>n</i>-pentane to 10-degree resolution at the CCSD­(T)-F12 level near the basis set limit. The data obtained have been decomposed by order of perturbation theory, excitation level, and same-spin vs opposite-spin character. A large number of approximate electronic structure methods have been considered, as well as several empirical dispersion corrections. Our chief conclusions are as follows: (a) the effect of dispersion is dominated by same-spin correlation (or triplet-pair correlation, from a different perspective); (b) singlet-pair correlation is important for the surface, but qualitatively very dissimilar to the dispersion component; (c) single and double excitations beyond third order are essentially unimportant for this surface; (d) connected triple excitations do play a role but are statistically very similar to the MP2 singlet-pair correlation; (e) the form of the damping function is crucial for good performance of empirical dispersion corrections; (f) at least in the lower-energy regions, SCS-MP2 and especially MP2.5 perform very well; (g) novel spin-component scaled double hybrid functionals such as DSD-PBEP86-D2 acquit themselves very well for this problem

What Can We Learn about Dispersion from the Conformer Surface of <i>n</i>‑Pentane?

In earlier work [Gruzman, D.; Karton, A.; Martin, J. M. L. J. Phys. Chem. A 2009, 113, 11974], we showed that conformer energies in alkanes (and other systems) are highly dispersion-driven and that uncorrected DFT functionals fail badly at reproducing them, while simple empirical dispersion corrections tend to overcorrect. To gain greater insight into the nature of the phenomenon, we have mapped the torsional surface of <i>n</i>-pentane to 10-degree resolution at the CCSD­(T)-F12 level near the basis set limit. The data obtained have been decomposed by order of perturbation theory, excitation level, and same-spin vs opposite-spin character. A large number of approximate electronic structure methods have been considered, as well as several empirical dispersion corrections. Our chief conclusions are as follows: (a) the effect of dispersion is dominated by same-spin correlation (or triplet-pair correlation, from a different perspective); (b) singlet-pair correlation is important for the surface, but qualitatively very dissimilar to the dispersion component; (c) single and double excitations beyond third order are essentially unimportant for this surface; (d) connected triple excitations do play a role but are statistically very similar to the MP2 singlet-pair correlation; (e) the form of the damping function is crucial for good performance of empirical dispersion corrections; (f) at least in the lower-energy regions, SCS-MP2 and especially MP2.5 perform very well; (g) novel spin-component scaled double hybrid functionals such as DSD-PBEP86-D2 acquit themselves very well for this problem

What Can We Learn about Dispersion from the Conformer Surface of <i>n</i>‑Pentane?

In earlier work [Gruzman, D.; Karton, A.; Martin, J. M. L. J. Phys. Chem. A 2009, 113, 11974], we showed that conformer energies in alkanes (and other systems) are highly dispersion-driven and that uncorrected DFT functionals fail badly at reproducing them, while simple empirical dispersion corrections tend to overcorrect. To gain greater insight into the nature of the phenomenon, we have mapped the torsional surface of <i>n</i>-pentane to 10-degree resolution at the CCSD­(T)-F12 level near the basis set limit. The data obtained have been decomposed by order of perturbation theory, excitation level, and same-spin vs opposite-spin character. A large number of approximate electronic structure methods have been considered, as well as several empirical dispersion corrections. Our chief conclusions are as follows: (a) the effect of dispersion is dominated by same-spin correlation (or triplet-pair correlation, from a different perspective); (b) singlet-pair correlation is important for the surface, but qualitatively very dissimilar to the dispersion component; (c) single and double excitations beyond third order are essentially unimportant for this surface; (d) connected triple excitations do play a role but are statistically very similar to the MP2 singlet-pair correlation; (e) the form of the damping function is crucial for good performance of empirical dispersion corrections; (f) at least in the lower-energy regions, SCS-MP2 and especially MP2.5 perform very well; (g) novel spin-component scaled double hybrid functionals such as DSD-PBEP86-D2 acquit themselves very well for this problem

Halogen Bonds: Benchmarks and Theoretical Analysis

We carried out an extensive survey of wave function and DFT methods to test their accuracy on geometries and dissociation energies of halogen bonds (XB). For that purpose, we built two benchmark sets (XB18 and XB51). Between the DFT methods, it was found that functionals with high exact exchange or long-range corrections were suitable for these dimers, especially M06-2X, ωB97XD, and double hybrids. Dispersion corrections tend to be detrimental, in spite of the fact that XB is considered a noncovalent interaction. Wave function techniques require heavy correlated methods (i.e., CCSD­(T)) or parametrized ones (SCS-MP2 or SCS­(MI)­MP2). Heavy basis sets are needed to obtain high accuracy, such as aVQZ or aVTZ+CP, and ideally a CBS extrapolation. Relativistic ECPs are also important, even for the bromine based dimers. In addition, we explored some XB with new theoretical tools, the NCI (“Non-Covalent Interactions”) method and the NOFF (“Natural Orbital Fukui Functions”)

Heats of Formation of Beryllium, Boron, Aluminum, and Silicon Re-examined by Means of W4 Theory

Benchmark total atomization energies (TAE0 values) were obtained, by means of our recent W4 theory [Karton, A.; Rabinowitz, E.; Martin, J. M. L.; Ruscic, B. J. Chem. Phys. 2006, 125, 144108], for the molecules Be2, BeF2, BeCl2, BH, BF, BH3, BHF2, B2H6, BF3, AlF, AlF3, AlCl3, SiH4, Si2H6, and SiF4. We were then able to deduce “semi-experimental” heats of formation for the elements beryllium, boron, aluminum, and silicon by combining the calculated TAE0 values with experimental heats of formation obtained from reactions that do not involve the species Be(g), B(g), Al(g), and Si(g). The elemental heats of formation are fundamental thermochemical quantities that are required whenever a molecular heat of formation has to be derived from a calculated binding energy. Our recommended [A(g)] values are Be 76.4 ± 0.6 kcal/mol, B 135.1 ± 0.2 kcal/mol, Al 80.2 ± 0.4 kcal/mol, and Si 107.2 ± 0.2 kcal/mol. (The corresponding values at 298.15 K are 77.4, 136.3, 80.8, and 108.2 kcal/mol, respectively.) The Be value is identical to the CODATA recommendation (but with half of the uncertainty), while the B, Al, and Si values represent substantial revisions from established earlier reference data. The revised B and Si values are in agreement with earlier semi-ab initio derivations but carry much smaller uncertainties

What Makes for a Bad Catalytic Cycle? A Theoretical Study on the Suzuki−Miyaura Reaction within the Energetic Span Model

The Suzuki−Miyaura cross-coupling reaction using PMe3, PPh3, and PtBu3 as ligands was studied theoretically with accurate density functional theory (DFT) methods and the Energetic Span Model. The energetic span model is a tool to compute catalytic turnover frequencies (TOF) from computationally obtained energy states. In this work the model is expanded to include turnover numbers (TON) and off-cycle intermediates. The results show that although the monophosphine route is the fastest pathway, the diphosphine cis route (accessible for small ligands) may also be reactive. The death sentence of the PMe3 catalyst is the possibility to reach the low energy trans diphosphine species, which substantially reduces the TON. In the PPh3 case, the formation of Pd0L3 was found to be the major drawback for efficient catalysis. The PtBu3 system is the most efficient of the three, as only the monophosphine mechanism is accessible

What Are the Ground State Structures of C₂₀ and C₂₄? An Explicitly Correlated Ab Initio Approach

A new benchmark study has been performed for six isomers of C₂₀ and four isomers of C₂₄ using explicitly correlated methods, together with coupled cluster theory with large basis sets and DFT with advanced functionals. The relative energy trends obtained are extremely sensitive to the methods used. Combining our best CCSD­(T)-MP2 difference with our best MP2 basis set limit, the dehydrocorannulene bowl is found to be the most stable for C₂₀, followed by the cage at about 8 kcal/mol, and the ring at about 46 kcal/mol. For C₂₄, the <i>D</i>_3<i>d</i> cage is found to be the most stable isomer, followed at only a few kilocalories per mole by dehydrocoronene, and at larger separations by then octahedral cage and the ring, respectively. This makes C₂₄ the smallest classical fullerene. The estimated residual basis set error of the estimated CCSD­(T) basis set limit is conservatively expected to be ±1 kcal/mol. In general, DFT exhibits large errors for relative energies with RMSD values in the 8–34 kcal/mol range. However, among the DFT functionals, the DSD-PBEP86-D3BJ double hybrid comes close to our best ab initio results, while the ωB97X-V range-separated hybrid is in semiquantitative agreement

Assessment of CCSD(T)-F12 Approximations and Basis Sets for Harmonic Vibrational Frequencies

We consider basis set convergence and the effect of various approximations to CCSD­(T)-F12 for a representative sample of harmonic frequencies (the HFREQ2014 set). CCSD­(T*)­(F12*)/cc-pVDZ-F12 offers a particularly favorable compromise between accuracy and computational cost: its RMSD <3 cm^–1 from the valence CCSD­(T) limit is actually less than the remaining discrepancy with the experimental value at the valence CCSD­(T) limit (about 5 cm^–1 RMSD). CCSD­(T)-F12a and CCSD­(T)-F12b appear to benefit from error compensation between CCSD and (T)

Insertion of Amines and Alcohols into Proton-Bound Dimers. A Density Functional Study

Insertion complexes of various bases with the protonated acetonitrile and acetone dimers have been studied using density functional methods including exact exchange contributions. The insertion mechanism has been investigated using an intrinsic reaction coordinate calculation. For some thermochemical quantities, calibration studies using larger basis sets and coupled cluster methods have been carried out. We find that B3LYP/cc-pVDZ will somewhat overestimate association energies due to basis set incompleteness error, which is partially compensated by an opposite error in the correlation treatment. B3LYP/4-21G will yield qualitatively correct structures, which PM3 and HF/4-21G generally do not, yielding instead asymmetric insertion complexes. The insertion energy increases with increasing proton affinity of the inserting base, while the association energy between the protonated central base and the ligands decreases. For the insertion complexes of the acetone dimer, the conformational equilibrium shifts from syn−syn to syn−anti with increasing proton affinity. The re geometries of protonated acetone dimer and its complexes are found to exhibit slight deviations from their intuitive symmetry that the calculations predict will be absent in the r0 geometries. Computed association and switching enthalpies are in very good agreement with experiment, while proton transfer enthalpies fare less well due to the change in hydrogen bond number involved. Geometries and vibrational frequencies for all structures considered are available as Supporting Information to the paper

Economical Post-CCSD(T) Computational Thermochemistry Protocol and Applications to Some Aromatic Compounds

To achieve a kilojoules-per-mole level of accuracy consistently in computational thermochemistry, the inclusion of post-CCSD(T) correlation effects cannot be avoided. Such effects are included in the W4 and HEAT computational thermochemistry protocols. The principal bottleneck in carrying out such calculations for larger systems is the evaluation of the T̂3−(T) term. We propose a cost-effective empirical approximation for this term that does not entail any reliance on experimental data. For first-row molecules, our W3.2lite protocol yields atomization energies with a 95% confidence interval of ∼0.4 kcal/mol at the expense of introducing two such parameters. W3.2lite has been successfully applied to aromatic and aliphatic hydrocarbons such as benzene, fulvene, phenyl radical, pyridine, furan, benzyne isomers, trans-butadiene, cyclobutene, [1.1.1]propellane, and bicyclo[1.1.1]pentane. The W3.2lite predictions for fulvene, phenyl radical, cyclobutene, and [1.1.1]propellane are impossible to reconcile with experiment and suggest that remeasurement may be in order