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The recently developed multiconfiguration pair-density functional theory (MC-PDFT) combines multiconfiguration wave function theory with a density functional that depends on the on-top pair density of an electronic system. In an MC-PDFT calculation, there are two steps: a conventional multiconfiguration self-consistent-field (MCSCF) calculation and a post-MCSCF evaluation of the energy with an on-top density functional. In this work, we present the details of the MC-PDFT algorithm that avoids steeply scaling steps that are present in other post-self-consistent-field multireference calculations of dynamic correlation energy. We demonstrate the favorable scaling by considering systems of H2 molecules with active spaces of several different sizes. We then apply the MC-PDFT method to calculate the heterolytic dissociation enthalpy of ferrocene. We find that MC-PDFT yields results that are at least as accurate as complete active space second-order perturbation theory and are more stable with respect to basis set, but at a fraction of the cost in both time and memory. [ABSTRACT FROM AUTHOR]

This work examines the question of how a Hubbard U correction to a local exchange–correlation functional compares with adding Hartree–Fock exchange to a local functional for both solid-state and molecular properties. We compute a solid-state property, namely the band gap, and thermochemical molecular properties, in particular, main-group bond energies, transition metal–ligand bond energies, and barrier heights, to elucidate whether the DFT+U method mimics hybrid DFT. We find that a calculation with a Hubbard U correction may or may not mimic a hybrid functional—depending on the atom, the subshell, and the property to which it is applied. For band gaps, we find that adding a Hubbard U correction to the valence d orbitals of transition metals increases the band gap, which thereby gets closer to the experimental value, while adding a Hubbard U correction to valence s or p orbitals of main-group elements need not always increase the band gap. For molecular thermochemistry, we find that adding a Hubbard U correction to a local density functional need not have the same effect as adding Hartree–Fock exchange to a local density functional. For example when compared to a DFT calculation with a local exchange-correlation functional, hybrid DFT increases the barrier height in all cases, but DFT+U does not always increase the barrier height. For the band gaps of transition metal monoxides, the Hubbard-corrected results lowered the mean errors significantly and were comparable to what could be achieved with a much more expensive hybrid functional, but for reaction barrier heights and bond energies of molecules, the Hubbard correction was found to lower the mean error by only approximately a kcal/mol. As part of the analysis, we also compare VASP and Gaussian 09 calculations for the same density functional. [ABSTRACT FROM AUTHOR]

Alkylperoxyl (RO 2) and hydroperoxyalkyl (QOOH) species are crucial intermediates produced during the autoignition of transport fuels, and consequently their thermodynamic and kinetic data are very important. Since these data are not available experimentally, we take up the challenge of computing them from first principles. We compare the 1,5 shifts in 2-methyl-4-heptylperoxyl radical and 3-methylcyclohexylperoxy radical to clarify the similarities and differences of the kinetics of chain alkanes and cycloalkanes in the fuel oxidation mechanism. In order to take account of the multistructural and coupled torsional anharmonicity, we employ multistructural canonical variational transition state theory (MS-VTST) with multidimensional tunneling to calculate rate constants. The calculations explicitly include all conformers: 6 and 154 for the two reactants, 7, 6, 1, and 1 for the four transition states, and 289, 282, 8, and 5 for the four products (which are the reactants for the reverse reactions). The effects of various factors on the thermodynamic data and calculated rate constants are evaluated in detail. We found that multistructural torsional anharmonicity has a very large effect on the rate constants. This work illustrates how modern density functional and dynamics methods make it possible to calculate accurate rate constants for both branched-chain and cycloalkane peroxy radicals and hydroperoxy species. [ABSTRACT FROM AUTHOR]

Screened-exchange hybrid density functionals are especially recommended for solid-state systems because they combine the advantages of hybrid functionals with the correct physics and lower computational cost associated with the attenuation of Hartree-Fock exchange at long range. We present a screenedexchange hybrid functional, M06-SX, that combines the functional form of the local revM06-L functional with a percentage of short-range nonlocal Hartree-Fock exchange. The M06-SX functional gives good results not only for a large set of training data but also for several databases quite different from the training data. The mean unsigned error (MUE) of the M06-SX functional is 2.85 kcal/mol for 418 atomic and molecular energies (AME418) in Minnesota Database 2019, which is better than all five other screened-exchange hybrid functionals tested in this work. The M06-SX functional also gives especially good results for semiconductor band gaps, molecular dissociation energies, noncovalent interactions, barrier heights, and electronic excitation energies excluding long-range charge transfer excitations. For the LC18 lattice constants database, the M06-SX functional gives an MUE of only 0.034 Å. Therefore, the M06-SX functional is well suited for studying molecular chemistry as well as solid-state physics. [ABSTRACT FROM AUTHOR]

We present a hybrid metageneralized-gradient-approximation functional, revM06, which is based on adding Hartree-Fock exchange to the revM06-L functional form. Compared with the original M06 suite of density functionals, the resulting revM06 functional has significantly improved across-the-board accuracy for both main-group and transition-metal chemistry. The revM06 functional improves on the M06-2X functional for main-group and transition-metal bond energies, atomic excitation energies, isomerization energies of large molecules, molecular structures, and both weakly and strongly correlated atomic and molecular data, and it shows a clear improvement over M06 and M06-2X for noncovalent interactions, including smoother potential curves for raregas dimers. The revM06 functional also predicts more accurate results than M06 and M06-2X for most of the outside-the-training-set test sets examined in this study. Therefore, the revM06 functional is well-suited for a broad range of chemical applications for both main-group and transition-metal elements. [ABSTRACT FROM AUTHOR]

The application of atomic layer deposition (ALD) to metal-organic frameworks (MOFs) offers a promising new approach to synthesize designer functional materials with atomic precision. While ALD on flat substrates is well established, the complexity of the pore architecture and surface chemistry in MOFs present new challenges. Through in situ synchrotron X-ray powder diffraction, we visualize how the deposited atoms are localized and redistribute within the MOF during ALD. We demonstrate that the ALD is regioselective, with preferential deposition of oxy-Zn(II) species within the small pores of NU-1000. Complementary density functional calculations indicate that this startling regioselectivity is driven by dispersion interactions associated with the preferential adsorption sites for the organometallic precursors prior to reaction. [ABSTRACT FROM AUTHOR]

The catalytic properties of the metal-organic framework Fe2(dobdc), containing open Fe(ll) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe01Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(lV)-oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C-H bonds of ethane. Kohn-Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C-H bond of ethane proceeds by a quintet single-state rt-attack pathway after the formation of highly reactive iron-oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N20 to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kj/mol, in contrast to 82 kj/mol for the slowest step in the iron(IV)-oxo catalytic mechanism. The energetics of the C-H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway. [ABSTRACT FROM AUTHOR]

This study examines whether the use of more accurate embedding potentials improves the convergence of quantum mechanics/molecular mechanics (QM/MM) models with respect to the size of the QM region. In conjunction with density functional theory calculations using the ωB97X-D functional, various embedding potentials including the TIP3P water model, the effective fragment potential (EFP), and semi-empirical methods (PM6, PM7, and DFTB) were used to simulate the deprotonation energies of solvated acid clusters. The calculations were performed on solvated neutral (HA) and cationic (HB+) acids clusters containing 160 and 480 water molecules using configurations sampled from molecular dynamics simulations. Consistently, the ωB97X-D/EFP model performed the best when using a minimal QM region size. The performance for the other potentials appears to be highly sensitive to the charge character of the acid/base pair. Neutral acids display the expected trend that semi-empirical methods generally perform better than TIP3P; however, an opposite trend was observed for the cationic acids. Additionally, electronic embedding provided an improvement over mechanical embedding for the cationic systems, but not the neutral acids. For the best performing ωB97X-D/EFP model, a QM region containing about 6% of the total number of solvent molecules is needed to approach within 10 kJ mol−1 of the pure QM result if the QM region was chosen based on the distance from the reaction centre. [ABSTRACT FROM AUTHOR]