[en] We evaluate the performance of standard and modified MPn procedures for a wide set of thermochemical and kinetic properties, including atomization energies, structural isomerization energies, conformational energies, and reaction barrier heights. The reference data are obtained at the CCSD(T)/CBS level by means of the Wn thermochemical protocols. We find that none of the MPn-based procedures show acceptable performance for the challenging W4-11 and BH76 databases. For the other thermochemical/kinetic databases, the MP2.5 and MP3.5 procedures provide the most attractive accuracy-to-computational cost ratios. The MP2.5 procedure results in a weighted-total-root-mean-square deviation (WTRMSD) of 3.4 kJ/mol, whilst the computationally more expensive MP3.5 procedure results in a WTRMSD of 1.9 kJ/mol (the same WTRMSD obtained for the CCSD(T) method in conjunction with a triple-zeta basis set). We also assess the performance of the computationally economical CCSD(T)/CBS(MP2) method, which provides the best overall performance for all the considered databases, including W4-11 and BH76.

[en] Highlights: • Study post-CCSD(T) contributions to the dissociation energy of C₂(¹${\mathrm{\Sigma }}_g^{+}$). • Examine contributions up to CCSDTQ567 in conjunction with basis sets up to cc-pV7Z. • At the CBS limit, post-CCSD(T) contributions add up to 0.427 kcal mol⁻¹. • Contributions up to CCSDTQ5(6) have to be obtained with relatively large basis sets. • Contributions beyond CCSDTQ5(6) are smaller than ~2 cm⁻¹. -- Abstract: We examine the basis-set convergence of post-CCSD(T) contributions to the dissociation energy of the highly multireference C₂(¹${\mathrm{\Sigma }}_g^{+}$) molecule. At the infinite basis set limit we obtain: CCSDT–CCSD(T)/cc-pV{6,7}Z = −2.268, CCSDT(Q)–CCSDT/cc-pV{6,7}Z = 3.420, CCSDTQ–CCSDT(Q)/cc-pV{5,6}Z = −1.151, CCSDTQ(5)–CCSDTQ/cc-pV{Q,5}Z = 0.412, CCSDTQ5–CCSDTQ(5)/cc-pV{T,Q}Z = −0.053, CCSDTQ5(6)–CCSDTQ5/cc-pV{D,T}Z = 0.060, CCSDTQ56–CCSDTQ5(6)/cc-pVTZ(1d) = 0.003, CCSDTQ56(7)–CCSDTQ56/cc-pVTZ(1d) = 0.002, and CCSDTQ567–CCSDTQ56(7)/cc-pVTZ(1d) = 0.001 kcal/mol. These post-CCSD(T) contributions add to 0.427 kcal/mol. Including the CCSD(T)/CBS energy, inner-shell, scalar relativistic, spin-orbit, DBOC, and ZPVE corrections from W4.3 theory results in a CCSDTQ567/CBS D₀ value of 144.08 kcal/mol, which agrees to within overlapping uncertainties with the experimental ATcT value of 144.006 ± 0.06 kcal/mol.

[en] The pursuit of a material capable of storing a high capacity of hydrogen (H₂) efficiently has prompted us to study the structural, electronic and H₂ storage properties of recently designed two-dimensional BN₂ nanosheets. Our spin-polarized density functional theory based calculations have revealed that the pristine BN₂ barely anchor H₂ molecules, however, alkali metal (AM) doping enhances the binding energies drastically. Van der Waals corrected energetics analysis established a uniform distribution of AMs over the BN₂ monolayers even at a high doping concentration of 12.50%, which ensure the reversibility of the systems. Bader charge analysis, Roby-Gould bond index method, and electron localization function isosurfaces conclude the transfer of charges from AMs to BN₂, which has resulted into strong ionic bonds between the former and the latter. The presence of partial positive charges on each of the AMs would adsorb multiple H₂ molecules with binding energies that are ideal for mobile H₂ storage applications. Considerably high H₂ storage capacities of 6.75%, 6.87% and 6.55% could be achieved with 3Li@BN₂, 3Na@BN₂ and 3K@BN₂ systems, respectively that guarantees the promise of AMs decorated BN₂ as a promising H₂ storage material.

[en] Highlights: • Generates database of energies & barrier heights for degenerate Cope rearrangements. • Benchmark energies are calculated at the CCSD(T)/CBS level via W1-F12 theory. • Evaluates the performance of a wide range of DFT, DHDFT, and ab initio methods. • Identifies the best performing DFT, DHDFT, and MP2-based methods. Shape-shifting molecules such as bullvalene undergo rapid structural reorganizations via degenerate Cope rearrangements. Here, we obtain accurate CCSD(T)/CBS barrier heights and reaction energies for a wide range of Cope rearrangements in substituted bullvalenes (C₁₀H₉R, R = NH₃, OH, CH₃, H, F, Cl, SH, and CN). We use this benchmark dataset to evaluate the performance of DFT and ab initio methods for the kinetics and thermodynamics of these reactions. The reaction barrier heights pose a significant challenge for DFT methods – the best methods attain root-mean-square deviations of 4.9 (BMK), 4.5 (PBE0), 4.2 (PW6B95), and 3.8 (B1B95) kJ mol⁻¹. Overall, only three DFT functionals (BMK, PW6B95, and MN12-SX) are able to surpass (or attain near) chemical accuracy for both barrier heights and reaction energies. In contrast, the double-hybrid DFT procedures ωB97X-2(LP), ωB97X-2(TQZ), PWPB95-D3, PBEQI-DH, and DSD-PBEB95-D3 give good-to-excellent performance.

[en] Highlights: • We calculate post-CCSD(T) atomization energies of n-alkanes by means of W4 theory. • Post-CCSD(T) contributions to the TAE increase linearly with the size of the n-alkane. • The post-CCSD(T) contributions to the TAE of hexane reach 0.65 kcal/mol. • Post-CCSD(T) contributions to the TAE of decane are expected to exceed 1 kcal/mol. The CCSD(T) method is often considered as the gold standard in quantum chemistry for single-reference systems. Using W4 and W4lite theories, we calculate post-CCSD(T) contributions to the total atomization energies (TAEs) of n-alkanes and show that they reach up to 0.65 kcal/mol for n-hexane. Furthermore, we find that post-CCSD(T) contributions increase linearly with the size of the n-alkane, indicating that they will reach ∼1 kcal/mol for n-decane (C₁₀H₂₂) and ∼2 kcal/mol for n-icosane (C₂₀H₄₂). These results are significant since today CCSD(T)/CBS-type methods are being applied to hydrocarbons of increasing size and are assumed to give TAEs with chemical accuracy for these systems.

[en] Highlights: • Investigates the conversion of α-tocopherol to α-tocopherylquinone. • This reaction is related to the antioxidant activity of vitamin E. • A water-catalysed ring-opening reaction plays a key role. • Designs an antioxidant with potentially enhanced antioxidant properties. The potent antioxidant α-tocopherol is known to trap two hydroxyl radicals leading to the formation of the experimentally observed α-tocopherylquinone product. Based on double-hybrid density functional theory calculations, we propose for the first time, a reaction mechanism for the conversion of α-tocopherol to α-tocopherylquinone. We find that a water-catalysed ring-opening reaction plays a key role in this conversion. The water catalysts act as proton shuttles facilitating the proton transfers and reducing the ring strain in the cyclic transition structures. On the basis of the proposed reaction mechanism, we proceed to design an antioxidant with potentially enhanced antioxidant properties.

[en] Highlights: • Investigates the H₂SO₄-catalysed reaction of glyoxal with ethanol to form hemiacetal. • The reaction energies and barrier height are calculated by means of the G4(MP2) theory. • In this reaction an intermolecular proton transfer is coupled with a CO bond formation. • An H₂SO₄ catalyst reduces the barrier for the RDS from 140.2 to 16.3 kJ/mol. We examine the reaction of ethanol with glyoxal to form hemiacetal by means of the high-level G4(MP2) procedure. In this reaction, an intermolecular proton transfer is coupled with the formation of a covalent CO bond between the two molecules. We find a novel catalytic reaction mechanism in which an H₂SO₄ catalyst reduces the barrier height from ∆H^‡₂₉₈ = 140.2 to 16.3 kJ mol⁻¹. It is well established that H₂SO₄ can effectively catalyse intramolecular proton transfers. This letter shows that H₂SO₄ can catalyse an intermolecular proton transfer that is coupled with a covalent bond formation.

[en] Highlights: • Evaluates the performance of force fields for 1811 C₆₀ isomerization energies. • Several potentials exhibit high statistical correlation with DFT reference values. • For REBO-II, ReaxFF, ABOP, and GAP-20 we obtain R² = 0.90–0.96. • The machine-learning GAP-20 potential outperforms conventional carbon potentials. We evaluate the performance of carbon force fields for 1811 C₆₀ PW6B95-D3/Def2-QZVP isomerization energies. Several force fields (most notably the machine-learning GAP-20 potential) exhibit a high statistical correlation with the DFT isomerization energies. Therefore, linear scaling of the isomerization energies can significantly improve the accuracy. The best scaled force fields attain mean-absolute deviations of 8.5 (GAP-20), 12.3 (LCBOP-I and REBO-II), and 13.3 (ABOP) kcal mol⁻¹, which translate to mean-absolute relative deviations of 4.7% (GAP-20), 6.5% (LCBOP-I), 6.6% (REBO-II) and 7.1% (ABOP). Therefore, these force fields offer a computationally economical way for exploring the relative energies of fullerenes.

[en] Coupled cluster calculations with all single and double excitations (CCSD) converge exceedingly slowly with the size of the one-particle basis set. We assess the performance of a number of approaches for obtaining CCSD correlation energies close to the complete basis-set limit in conjunction with relatively small DZ and TZ basis sets. These include global and system-dependent extrapolations based on the A + B/L"α two-point extrapolation formula, and the well-known additivity approach that uses an MP2-based basis-set-correction term. We show that the basis set convergence rate can change dramatically between different systems(e.g.it is slower for molecules with polar bonds and/or second-row elements). The system-dependent basis-set extrapolation scheme, in which unique basis-set extrapolation exponents for each system are obtained from lower-cost MP2 calculations, significantly accelerates the basis-set convergence relative to the global extrapolations. Nevertheless, we find that the simple MP2-based basis-set additivity scheme outperforms the extrapolation approaches. For example, the following root-mean-squared deviations are obtained for the 140 basis-set limit CCSD atomization energies in the W4-11 database: 9.1 (global extrapolation), 3.7 (system-dependent extrapolation), and 2.4 (additivity scheme) kJ mol"–"1. The CCSD energy in these approximations is obtained from basis sets of up to TZ quality and the latter two approaches require additional MP2 calculations with basis sets of up to QZ quality. We also assess the performance of the basis-set extrapolations and additivity schemes for a set of 20 basis-set limit CCSD atomization energies of larger molecules including amino acids, DNA/RNA bases, aromatic compounds, and platonic hydrocarbon cages. We obtain the following RMSDs for the above methods: 10.2 (global extrapolation), 5.7 (system-dependent extrapolation), and 2.9 (additivity scheme) kJ mol"–"1

[en] Highlights: • D_5h → D_10h isomerization in C₁₀ is investigated at the CCSDT(Q)/CBS level. • This isomerization has a large contribution from higher- order triple excitations. • Accurate treatment of post-CCSD(T) correlation and ZPVE effects is needed. • CCSD(T) composite methods erroneously predict that the D_10h structure is more stable. The D_5h → D_10h isomerization in the C₁₀ carbon cluster is investigated at the relativistic, all-electron CCSDT(Q)/CBS level. Previous high-level studies examined this isomerization at the valence CCSD(T)/CBS level. We show that capturing this isomerization energy requires accurate treatment of the CCSD(T)/CBS, post-CCSD(T), core-valence, scalar relativistic, diagonal Born–Oppenheimer, and zero-point vibrational energy components. Combining these components shows that the two structures are practically isoenergetic at 0 K (i.e., the D_5h structure is more stable by merely +0.100 kcal mol⁻¹). We also show that computationally economical composite protocols erroneously predict that the D_10h structure is energetically more stable at 0 K.