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A complex GMRes (generalized minimum residual) algorithm is presented and used to solve dense systems of linear equations arising in variational basis-set approaches to quantum-mechanical reactive scattering. The examples presented correspond to physical solutions of the Schrödinger equation for the reactions O+HD→OH+D, D+H2→HD+H, and H+H2→H2+H. It is shown that the computational effort for solution with GMRes depends upon both the dimension of the linear system and the total energy of the reaction. In several cases with dimensions in the range 1110–5632, GMRes outperforms the LAPACK direct solver, with speedups for the linear equation solution as large as a factor of 23. In other cases, the iterative algorithm does not converge within a reasonable time. These convergence differences can be correlated with ‘‘indices of diagonal dominance,’’ which we define in detail and which are relatively easy to compute. Furthermore, we find that for a given energy, the computational effort for GMRes can vary with dimension as favorably as M1.7, where M is the dimension of the linear system, whereas the computer time for a direct method is approximately proportional to the cube of the dimension of the linear system. [ABSTRACT FROM AUTHOR]

An optimized integration scheme for calculating vibrational–rotational partition functions by the Fourier path-integral method, as presented in the previous paper [R. Q. Topper and D. G. Truhlar, J. Chem. Phys. XX, ▪▪▪▪ (19▪▪)] is applied to a three-dimensional test case involving the coupled vibrational and rotational motions of a diatomic HCl molecule in Cartesian coordinates. Converged partition functions are calculated by the new Fourier path-integral Monte Carlo scheme and by standard variational methods, and the two sets of results are compared. We obtain good agreement (∼2%) between the two methods over a range of a factor of 20 in temperature. [ABSTRACT FROM AUTHOR]

In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero-point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum-energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential-energy functions we need to take account of these factors and—especially at lower temperatures—to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential-energy surface than is required for conventional transition-state theory. In the present article we show how the vibrational and entropic effects of variational transition-state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH+H2, CH3+H2, and Cl+CH4 and some of their isotopic analogs. We find for Cl+CH4→HCl+CH3 that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry. [ABSTRACT FROM AUTHOR]

The isotopic fractionation factor in the monohydrated gas-phase cluster Cl-(H2O), i.e., the equilibrium constant for Cl-(H2O)(g)+D2O(g)⇄Cl-(D2O)(g) +H2O(g), is used to test models used for solution-phase simulations and to test semiempirical and ab initio molecular orbital theory for their detailed structural and vibrational predictions for both the solute–solvent bond properties and the interaction-induced intramolecular changes in water itself. The isotope effect is studied at a consistent level of vibration–rotation theory, i.e., the harmonic-oscillator-rigid-rotator approximation, using four different kinds of approach, namely extended-basis-set ab initio electronic structure calculations, both (i) with and (ii) without electron correlation, (iii) semiempirical molecular orbital theory at the level of neglect of diatomic differential overlap, and (iv) analytic force fields based on site–site interactions. The calculations of type (i) show good convergence and are compared both to experiment, which presumably tests the harmonic approximation, and to the results of efforts (ii), (iii), and (iv), which presumably tests the structural and vibrational properties predicted by these more approximate approaches. We find significant effects of anharmonicity and electron correlation; semiempirical molecular orbital theory does remarkably well; and there is a wide variation in predictions among the 25 analytic force fields tested. Finally we combine the well converged ab initio results for the properties of the well, the vibrational red and blue shifts, and the isotope effect on the equilibrium constant with the previous ab initio calculations of Dacre for the repulsive interactions to obtain a new interaction potential for chloride ion with nonrigid water that also predicts a more accurate enthalpy of complexation and binding energy than do the chloride–water potentials available in the literature. [ABSTRACT FROM AUTHOR]

We report converged quantum mechanical calculations of scattering matrices and transition probabilities for the reaction of H with H2 with total angular momentum 0, 1, and 4 as functions of total energy in the range 0.85–1.15 eV on an accurate potential energy surface. These calculations show energy dependences that may be attributed to dynamical resonances with vibrational quantum numbers (100 0) and (111 0). The resonance structure is illustrated with Argand diagrams, and we present state-to-state reactive collision delay times and lifetimes. For J=0, 1, and 4, we found the lowest-energy H3 resonance at total energies of 0.983, 0.985, and 1.01 eV, respectively, with lifetimes of about 16–17 fs. For J=1 and 4 there is a higher-energy resonance at 1.10–1.11 eV; for J=1 the lifetime is about 4 fs and for J=4 it is about 1 fs. [ABSTRACT FROM AUTHOR]

We study low-energy quantal phenomena in the rearrangement of three-atom systems composed of H and D. All calculations are carried out on the double many-body expansion potential energy surface for the hydrogen trimer. The unimolecular rearrangements of van der Waals molecules, such as D···H2→HD···H, are studied as a model for the exchange transfer reaction in condensed phases, and the gas-phase bimolecular reactions, such as D+H2→HD+H, are studied to probe the limiting low-temperature threshold behavior, which is compared to that predicted by quantum mechanical threshold laws. The reaction rates are studied down to temperatures of 10-3 K. We also predict the spectroscopic tunneling shift on the lowest energy levels of the H···H2 complex. [ABSTRACT FROM AUTHOR]

We present a sequence of three successively improved new semiempirical potential energy surfaces for the reaction CH3+H2→CH4+H. The semiempirical calibration is based on ab initio electronic structure calculations and experimental thermochemical data, vibrational frequencies, reaction rate constants, Arrhenius parameters, and kinetic isotope effects (KIE’s). To compare to the experimental kinetic data we apply variational transition state theory and semiclassical estimates of tunneling probabilities. We also provide detailed factorization analyses of the KIE’s to illustrate the way in which various surface features contribute to the overall KIE’s, and we discuss the substantial difficulties in attributing specific kinetic results to isolated potential energy surface features. Each of the three new surfaces, called J1, J2, and J3, has a thinner barrier than the one before. In addition, we provide one example, called surface J2A, showing the effect of making the barrier even thinner than on the best surface. The best surface yields rate constants for the forward and reverse reaction, activation energies, and KIE’s that are consistent with most of the available experimental data. [ABSTRACT FROM AUTHOR]

We examine the question, given a set of state-to-state rate constants k( j=0 → j’|T) for collision-induced rotational transitions in a diatomic molecule, where j and j’ are initial and final rotational quantum numbers and T is the translational temperature, can one use scaling analyses to predict a full set of j → j’ rate constants? To answer this we consider a rigid rotator model of CO in a bath of Ar at 500 K, and we calculate accurate quasiclassical k( j → j’|T) for j=0, 10, and 20 and j’=0–29. These are used to test the energy sudden (ES) and energy-corrected sudden (ECS) scaling procedures. Both procedures are used to predict the j=10 and j=20 rate constants from the j=0 values. The ES procedure, which has no adjustable parameters, overestimates the rates out of excited states by a factor of about 1.5 with a rms error of about 60%. The ECS procedure, in contrast, when the one parameter bc (a critical impact parameter) is about 1.75–2.0 Å, yields excellent excited-state rates on the average and has a rms error of less than 20%. The value of bc can be estimated by a weighted average impact parameter leading to inelastic collisions from a j=0 initial state. [ABSTRACT FROM AUTHOR]

The small-curvature semiclassical adiabatic (SCSA) approximation, which is based on a reaction-path Hamiltonian, is used to calculate the rate of interconversion of the ammonia invertomers. In these calculations, we employ two realistic potential energy surfaces for which accurate tunneling splittings have been calculated previously. The semiclassical method is shown to be accurate within 16% for this prototype quantal isomerization rate. [ABSTRACT FROM AUTHOR]

DFT and coupled-cluster calculations show that the B2N2O4 molecule has a triplet ground state and therefore is paramagnetic. [ABSTRACT FROM AUTHOR]

Review: [117 refs. [ABSTRACT FROM AUTHOR]

Criegee intermediates (i.e., carbonyl oxides with two radical sites) are known to be important atmospheric reagents; however, our knowledge of their reaction kinetics is still limited. Although experimental methods have been developed to directly measure the reaction rate constants of stabilized Criegee intermediates, the experimental results cover limited temperature ranges and do not completely agree well with one another. Here we investigate the unimolecular reaction of acetone oxide [(CH3)2COO] and its bimolecular reaction with H2O to obtain rate constants with quantitative accuracy comparable to experimental accuracy. We do this by using CCSDT(Q)/CBS//CCSD(T)-F12a/DZ-F12 benchmark results to select and validate exchange-correlation functionals, which are then used for direct dynamics calculations by variational transition state theory with small-curvature tunneling and torsional and high-frequency anharmonicity. We find that tunneling is very significant in the unimolecular reaction of (CH3)2COO and its bimolecular reaction with H2O. We show that the atmospheric lifetimes of (CH3)2COO depend on temperature and that the unimolecular reaction of (CH3)2COO is the dominant decay mode above 240 K, while the (CH3)2COO + SO2 reaction can compete with the corresponding unimolecular reaction below 240 K when the SO2 concentration is 9 × 1010 molecules per cubic centimeter. We also find that experimental results may not be sufficiently accurate for the unimolecular reaction of (CH3)2COO above 310 K. Not only does the present investigation provide insights into the decay of (CH3)2COO in the atmosphere, but it also provides an illustration of how to use theoretical methods to predict quantitative rate constants of medium-sized Criegee intermediates. [ABSTRACT FROM AUTHOR]

Spin-state energetics are important for understanding properties that involve more than one spin state, for example, catalysis occurring on two or more potential energy surfaces corresponding to different electronic spins. Very often, multiple-spin processes involve transition-metal compounds, and therefore, it is important to understand the electronic structure and energetics of such compounds in different spin states. In this work, we benchmark relative spin-state energies of FeF2 with respect to the quintet ground spin state using both single-configurational and multiconfigurational methods, and we examine how they are affected by the binding of ethane and ethylene to the iron center. We also benchmark the binding energies of the complexes. The single-configurational methods used in this work are the Hartree-Fock method, 32 exchange-correlation functionals, and the CCSD(T) coupled-cluster method in both restricted and unrestricted formalisms. The multiconfigurational methods that have been used are CASSCF, CASPT2, CASPT3, MRCI, MRCI+Q, and MR-ACPF. The spin-state splitting energies depend on the functional chosen, and of the 32 exchange-correlation functionals investigated here, we find that for the septet and spin-projected triplet states of FeF2 the M06 functional is the best when compared to our best estimates from multireference calculations. If all nine excitation energies are considered, where there are three excited spin states (singlet, triplet, and septet) for each of the three systems (FeF2, FeF2. . .ethane, and FeF2. . .ethylene), the three best-performing functionals are HLE16, SOGGA11-X, and M06-2X. We find that the binding of ethane perturbs the relative spin-state energy of FeF2 by only a small amount, but the stronger binding of ethylene has a larger effect. For the spin-state splitting energies of FeF2 using single-reference CCSD(T), we find that the predicted results depend very strongly on precisely how the calculations are done, in particular, on the spin-restricted or spin-unrestricted character of the SCF reference state, which can differ even by around 50 kcal/mol for the SCF reference state and the subsequent CCSD(T) calculations. Upon analyzing the wave functions of both the spin-restricted or spin-unrestricted formalisms, we find that the lowest-energy singlet and triplet states of the complexes, just like FeF2 in isolation, can have more unpaired electrons than they are usually assumed to have, i.e., they can be hyper open-shell electronic configurations, and this can significantly lower the energy. [ABSTRACT FROM AUTHOR]

Recently we have developed the dual-functional Tamm-Dancoff approximation (DF-TDA) method. DF-TDA is an alternative to linear-response time-dependent density functional theory (LR-TDDFT) with the advantage of providing a correct double-cone topology of S1/S0 conical intersections. In the DF-TDA method, we employ different functionals, which are denoted G and F, for orbital optimization and Hamiltonian construction. We use the notation DF-TDA/G:F. In the current work, we propose that G be the same as F except for having 100% Hartree-Fock exchange. We use the notation F100 to denote functional F with this modification. A motivation for this is that functionals with 100% Hartree-Fock exchange are one-electron self-interaction-free. Here we validate the use of F100/M06 to compute vertical excitation energies and the global potential energy surface of ammonia near a conical intersection to further validate the F100 method for photochemical problems. [ABSTRACT FROM AUTHOR]

We investigate the performance of multiconfiguration pair-density functional theory (MC-PDFT) and complete active space second-order perturbation theory for computing the bond dissociation energies of the diatomic molecules FeC, NiC, FeS, NiS, FeSe, and NiSe, for which accurate experimental data have become recently available [Matthew, D. J.; Tieu, E.; Morse, M. D. J. Chem. Phys. 2017, 146, 144310-144320]. We use three correlated participating orbital (CPO) schemes (nominal, moderate, and extended) to define the active spaces, and we consider both the complete active space (CAS) and the separated-pair (SP) schemes to specify the configurations included for a given active space. We found that the moderate SP-PDFT scheme with the tPBE on-top density functional has the smallest mean unsigned error (MUE) of the methods considered. This level of theory provides a balanced treatment of the static and dynamic correlation energies for the studied systems. This is encouraging because the method is low in cost even for much more complicated systems. [ABSTRACT FROM AUTHOR]

The reaction between H and benzene is a prototype for reactions of radicals with aromatic hydrocarbons. Here we report calculations of the reaction rate constants and the branching ratios of the two channels of the reaction (H addition and H abstraction) over a wide temperature and pressure range. Our calculations, obtained with an accurate potential energy surface, are based on variational transition-state theory for the high-pressure limit of the addition reaction and for the abstraction reaction and on system-specific quantum Rice-Ramsperger-Kassel theory calibrated by variational transition-state theory for pressure effects on the addition reaction. The latter is a very convenient way to include variational effects, corner-cutting tunneling, and anharmonicity in falloff calculations. Our results are in very good agreement with the limited experimental data and show the importance of including pressure effects in the temperature interval where the mechanism changes from addition to abstraction. We found a negative temperature effect of the total reaction rate constants at 1 atm pressure in the temperature region where experimental data are missing and accurate theoretical data were previously missing as well. We also calculated the H + C6H6/C6D6 and D + C6H6/C6D6 kinetic isotope effects, and we compared our H + C6H6 results to previous theoretical data for H + toluene. We report a very novel nonmonotonic dependence of the kinetic isotope effect on temperature. A particularly striking effect is the prediction of a negative temperature dependence of the total rate constant over 300-500 K wide temperature ranges, depending on the pressure but generally in the range from 600 to 1700 K, which includes the temperature range of ignition in gasoline engines, which is important because aromatics are important components of common fuels. [ABSTRACT FROM AUTHOR]

Excited spin states are important for reactivity, catalysis, and magnetic applications. This work examines the relative energies of the spin states of O atom, Fe2+ ion, and FeF2 and characterizes their excited spin states. Both single-configuration and multireference methods are used to establish the character of the lowest singlet excited state of all three systems and the lowest triplet excited state of Fe2+ and FeF2. We find that the conventional representation of the orbital occupancies is incorrect in that the states have more unpaired electrons than the minimum number required by their total electron spin quantum number. In particular, we find that, for a given spin state, an electronic configuration with more than 2S unpaired electrons is more stable than the configuration with 2S unpaired electrons (where S is the spin of the system). For instance, triplet FeF2 with four unpaired electrons is lower in energy than triplet FeF2 with two unpaired electrons. Such highly open-shell configurations are labeled as hyper open-shell electronic configurations in this work and are compared to ordinary open-shell or closed-shell electronic configurations. The hyper open-shell states considered in this work are especially interesting because, unlike typical biradicals and polyradicals, the unpaired electrons are all on the same center. This work shows that the conventional perspective on spin-state energetic that usually assumes ordinary open shells for single-centered radicals needs modification to take into account, whenever possible, hyper open-shell configurations as well. [ABSTRACT FROM AUTHOR]

Analytic gradients are important for efficient calculations of stationary points on potential energy surfaces, for interpreting spectroscopic observations, and for efficient direct dynamics simulations. For excited electronic states, as are involved in UV–Vis spectroscopy and photochemistry, analytic gradients are readily available and often affordable for calculations using a state-averaged complete active space self-consistent-field (SA-CASSCF) wave function. However, in most cases, a post-SA-CASSCF step is necessary for quantitative accuracy, and such calculations are often too expensive if carried out by perturbation theory or configuration interaction. In this work, we present the analytic gradients for multiconfiguration pair-density functional theory based on SA-CASSCF wave functions, which is a more affordable alternative. A test set of molecules has been studied with this method, and the stationary geometries and energetics are compared to values in the literature as obtained by other methods. Excited-state geometries computed with state-averaged pair-density functional theory have similar accuracy to those from complete active space perturbation theory at the second-order. [ABSTRACT FROM AUTHOR]

We have employed extended multiconfiguration quasidegenerate perturbation theory, fourfold-way diabatic molecular orbitals, and configurational uniformity to develop a global three-state diabatic representation of the potential energy surfaces and their couplings for the electronically nonadiabatic reaction OH* + H2 → H2O + H, where * denotes electronic excitation to the A 2Σ+ state. To achieve sign consistency of the computed diabatic couplings, we developed a graphics processing unit-accelerated algorithm called the cluster-growing algorithm. Having obtained consistent signs of the diabatic couplings, we fit the diabatic matrix elements (which consist of the diabatic potentials and the diabatic couplings) to analytic representations. Adiabatic potential energy surfaces are generated by diagonalizing the 3 × 3 diabatic potential energy matrix. The comparisons between the fitted and computed diabatic matrix elements and between the originally computed adiabatic potential energy surfaces and those generated from the fits indicate that the current fit is accurate enough for dynamical studies, and it may be used for quantal or semiclassical dynamics calculations. [ABSTRACT FROM AUTHOR]

Kohn-Sham density functional theory is in principle an exact formulation of quantum mechanical electronic structure theory, but in practice we have to rely on approximate exchange-correlation (xc) functionals. The objective of our work has been to design an xc functional with broad accuracy across as wide an expanse of chemistry and physics as possible, leading--as a long-range goal--to a functional with good accuracy for all problems, i.e. a universal functional. To guide our path towards that goal and to measure our progress, we have developed--building on earlier work of our group--a set of databases of reference data for a variety of energetic and structural properties in chemistry and physics. These databases include energies of molecular processes, such as atomization, complexation, proton addition and ionization; they also include molecular geometries and solid-state lattice constants, chemical reaction barrier heights, and cohesive energies and band gaps of solids. For this paper, we gather many of these databases into four comprehensive databases, two with 384 energetic data for chemistry and solid-state physics and another two with 68 structural data for chemistry and solid-state physics, and we test two wave function methods and 77 density functionals (12 Minnesota meta functionals and 65 others) in a consistent way across this same broad set of data. We especially highlight the Minnesota density functionals, but the results have broader implications in that one may see the successes and failures of many kinds of density functionals when they are all applied to the same data. Therefore, the results provide a status report on the quest for a universal functional. [ABSTRACT FROM AUTHOR]

The article offers information related to chemical reactivity inverse solvent design of chemical compounds.

Hydroperoxides are formed in the atmospheric oxidation of volatile organic compounds, in the combustion autoxidation of fuel, in the cold environment of the interstellar medium, and also in some catalytic reactions. They play crucial roles in the formation and aging of secondary organic aerosols and in fuel autoignition. However, the concentration of organic hydroperoxides is seldom measured, and typical estimates have large uncertainties. In this work, we developed a mild and environmental- friendly method for the synthesis of alkyl hydroperoxides (ROOH) with various structures, and we systematically measured the absolute photoionization cross-sections (PICSs) of the ROOHs using synchrotron vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS). A chemical titration method was combined with an SVUV-PIMS measurement to obtain the PICS of 4-hydroperoxy-2-pentanone, a typical molecule for combustion and atmospheric autoxidation ketohydroperoxides (KHPs). We found that organic hydroperoxide cations are largely dissociated by loss of OOH. This fingerprint was used for the identification and accurate quantification of the organic peroxides, and it can therefore be used to improve models for autoxidation chemistry. The synthesis method and photoionization dataset for organic hydroperoxides are useful for studying the chemistry of hydroperoxides and the reaction kinetics of the hydroperoxy radicals and for developing and evaluating kinetic models for the atmospheric autoxidation and combustion autoxidation of the organic compounds. [ABSTRACT FROM AUTHOR]

Bond dissociation is a fundamental chemical reaction, and the first principles modeling of the kinetics of dissociation reactions with a monotonically increasing potential energy along the dissociation coordinate presents a challenge not only for modern electronic structure methods but also for kinetics theory. In this work, we use multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) to compute the highpressure limit dissociation rate constant of tetrafluoroethylene (C2F4), in which the potential energies are computed by direct dynamics with the M08-HX exchange correlation functional. To treat the pressure dependence of the unimolecular rate constants, we use the recently developed system-specific quantum Rice--Ramsperger--Kassel theory. The calculations are carried out by direct dynamics using an exchange correlation functional validated against calculations that go beyond coupled-cluster theory with single, double, and triple excitations. Our computed dissociation rate constants agree well with the recent experimental measurements. [ABSTRACT FROM AUTHOR]

Criegee intermediates are produced in the ozonolysis of unsaturated hydrocarbons in the troposphere, and understanding their fate is a prerequisite to modeling climate-controlling atmospheric aerosol formation. Although some experimental and theoretical rate data are available, they are incomplete and partially inconsistent, and they do not cover the tropospheric temperature range, Here, we report quantum chemical rate constants for the reactions of stabilized formaldehyde oxide (CH2OO) and acetaldehyde oxide (syn-CH3CHOO and anti-CH3CHOO) with H2O and for their unimolecular reactions. Our results are obtained by combining post- CCSD(T) electronic structure benchmarks, validated density functional theory potential energy surfaces, and multipath variational transition state theory with multidimensional tunneling, coupled-torsions anharmonicity, and high-frequency anharmonicity. We consider two different types of reaction mechanisms for the bimolecular reactions, namely, (i) addition-coupled hydrogen transfer and (ii) double hydrogen atom transfer (DHAT). First, we show that the MN15-L exchange-correlation functional has kJ/mol accuracy for the CH2OO + H2O and syn-CH3CHOO + H2O reactions. Then we show that, due to tunneling, the DHAT mechanism is especially important in the syn-CH3CHOO + H2O reaction. We show that the dominant pathways for reactions of Criegee intermediates depend on altitude. The results we obtain eliminate the discrepancy between experiment and theory under those conditions where experimental results are available, and we make predictions for the full range of temperatures and pressures encountered in the troposphere and stratosphere. The present results are an important cog in clarifying the atmospheric fate and oxidation processes of Criegee intermediates, and they also show how theoretical methods can provide reliable rate data for complex atmospheric processes. [ABSTRACT FROM AUTHOR]

We report converged quantum mechanical reaction probabilities for O+H2(v=0,1)→OH+H for zero total angular momentum as obtained by an L2 expansion of the reactive amplitude density. These provide a benchmark for testing approximate dynamical theories, and this is illustrated by comparisons to centrifugal sudden distorted wave and least-action calculations and vibrationally adiabatic threshold energies. [ABSTRACT FROM AUTHOR]

The article reflects on the developments of the 2006 issue of the periodical "Theoretical Chemistry Accounts." The recent issue presented 33 articles which cover areas related to theoretical chemistry by modern theoretical chemists. Senior advisers suggested contributors including Biman Bagchi, Vincenzo Barone, Charles Brooks and other colleagues.

Photochemical reactions often involve states that are closely coupled due to near degeneracies, for example by proximity to conical intersections. Therefore, a multistate method is used to accurately describe these states; for example, ordinary perturbation theory is replaced by quasidegenerate perturbation theory. Multiconfiguration pair-density functional theory (MC-PDFT) provides an efficient way to approximate the full dynamical correlation energy of strongly correlated systems, and we recently proposed compressed multistate pair-density functional theory (CMS-PDFT) to treat closely coupled states. In the present paper, we report the implementation of analytic gradients for CMS-PDFT in both OpenMolcas and PySCF, and we illustrate the use of these gradients by applying the method to the excited states of formaldehyde and phenol. [ABSTRACT FROM AUTHOR]

Pressure-dependent reactions are ubiquitous in combustion and atmospheric chemistry. We employ a new calibration procedure for quantum Rice-Ramsperger-Kassel (QRRK) unimolecular rate theory within a chemical activation mechanism to calculate the pressure-falloff effect of a radical association with an aromatic ring. The new theoretical framework is applied to the reaction of H with toluene, which is a prototypical reaction in the combustion chemistry of aromatic hydrocarbons present in most fuels. Both the hydrogen abstraction reactions and the hydrogen addition reactions are calculated. Our system-specific (SS) QRRK approach is adjusted with SS parameters to agree with multistructural canonical variational transition state theory with multidimensional tunneling (MS-CVT/SCT) at the high-pressure limit. The new method avoids the need for the usual empirical estimations of the QRRK parameters, and it eliminates the need for variational transition state theory calculations as a function of energy, although in this first application we do validate the falloff curves by comparing SS-QRRK results without tunneling to multistructural microcanonical variational transition state theory (MS-µVT) rate constants without tunneling. At low temperatures, the two approaches agree well with each other, but at high temperatures, SS-QRRK tends to overestimate falloff slightly. We also show that the variational effect is important in computing the energy-resolved rate constants. Multiple-structure anharmonicity, torsional-potential anharmonicity, and high-frequency-mode vibrational anharmonicity are all included in the rate computations, and torsional anharmonicity effects on the density of states are investigated. Branching fractions, which are both temperature- and pressure-dependent (and for which only limited data is available from experiment), are predicted as a function of pressure. [ABSTRACT FROM AUTHOR]

The kinetics of the hydrogen abstraction from 2-butanol by hydroxyl radical have been studied using multipath variational transition-state theory with the multidimensional small curvature tunneling approximation. The rate constants for each of the five hydrogen abstraction sites (C1, C2, C3, C4, and O) and the overall reaction have been computed by direct dynamics based on M08-HX/6-311+G(2df,2p) electronic structure calculations. We show that multistructural torsional anharmonicity, anharmonicity differences of high-frequency modes between the transition structures and the reactants, and reaction-path dependence of multiple reaction paths are all important factors for determining accurate reaction rates and branching fractions for this problem. The reaction barrier heights for abstraction from various sites follow the order C2 < C3 < C4 < C1 < O, but the reactivities of the various sites do not precisely follow the inverse order of barrier heights, and the order of reactivities depends on temperature. The abstractions from C2 and C3 have the largest contribution to the total reaction rate from 200 to 2000 K. [ABSTRACT FROM AUTHOR]

We present a global ground-state potential energy surface (PES) for the triplet spin state of O4 that is suitable for treating high-energy vibrational–rotational energy transfer and collision-induced dissociation in electronically adiabatic spin-conserving O2–O2 collisions. The surface is based on MS-CASPT2/maug-cc-pVTZ electronic structure calculations with scaled external correlation; the active space has 16 electrons in 12 orbitals. The global ground-state potential energy surface was fitted by a many-body approach with an accurate O–O pairwise interaction and a fit of the many-body interaction potential to 10 180 electronic structure data points. The many-body fit is based on permutationally invariant polynomials in terms of bond-order functions of the six interatomic distances; the bond-order functions are mixed exponential–Gaussian functions. The geometries calculated and used for the fit include geometry scans corresponding to dissociative and vibrationally excited diatom–diatom collisions of O2, scans corresponding to O3 interacting with O, additional geometries identified by running trajectories, and geometries along linear synchronous transit paths connecting randomly selected points. The global O4 PES includes subsurfaces describing the interaction of diatomic molecules with other diatomic molecules or interactions of triatomic molecules and an atom. The interaction of ozone with a ground-state oxygen atom occurs on the triplet O4 surface, and our surface includes high-energy points with O3–O geometries as well as O2–O2 geometries and O2–O–O geometries. [ABSTRACT FROM AUTHOR]

Singlet-triplet gaps in diradical organic π-systems are of interest in many applications. In this study, we calculate them in a series of molecules, including cyclobutadiene and its derivatives and cyclopentadienyl cation, by using correlated participating orbitals within the complete active space (CAS) and restricted active space (RAS) self-consistent field frameworks, followed by second-order perturbation theory (CASPT2 and RASPT2). These calculations are evaluated by comparison with the results of doubly electron-attached (DEA) equation-of-motion (EOM) coupled-cluster (CC) calculations with up to 4-particle-2-hole (4p-2h) excitations. We find active spaces that can accurately reproduce the DEA-EOMCC(4p-2h) data while being small enough to be applicable to larger organic diradicals. [ABSTRACT FROM AUTHOR]

Free energy calculations for eight enol isomers of malonaldehyde (MA) and simulation of the ultraviolet (UV) absorption spectrum in both the gas phase and water (pH = 3, where the molecule exists in neutral undeprotonated form) show that in water the two s-trans nonchelated enol conformers of MA become thermodynamically more stable than the internally hydrogen-bonded ("chelated enol") conformer (CE). The pure CE conformer in water has a slightly red-shifted UV spectrum with respect to that in the gas phase, but the blue-shifted spectrum observed in water at pH 3 is dominated by solvent-stabilized conformations that have negligible populations in the gas phase. Density functional calculations with the solvation model based on density (SMD) and an ensemble-averaged vertical excitation model explain the experimental observations in detail. [ABSTRACT FROM AUTHOR]

The article discusses the quantum chemical characterization of properties and reactivities of metal-organic frameworks. It talks about different electronic structure methods, properties of metal-organic frameworks, and spectroscopic properties of metal-organic frameworks (MOFs). It mentions Monte Carlo simulation of molecular structures and prediction of synthetic pathways for MOFs.

A physically realistic treatmentof solvatochromic shifts in liquid-phaseelectronic absorption spectra requires a proper account for variousshort- and long-range equilibrium and nonequilibrium solute–solventinteractions. The present article demonstrates that such a treatmentcan be accomplished using a mixed discrete–continuum approachbased on the two-time-scale self-consistent state-specific verticalexcitation model (called VEM) for electronic excitation in solution.We apply this mixed approach in combination with time-dependent densityfunctional theory to compute UV/vis absorption spectra in solutionfor the n → π* (1A2) transitionfor acetone in methanol and in water, the π → π*(1A1) transition for para-nitroaniline(PNA) in methanol and in water, the n → π* (1B1) transition for pyridine in water, and the n →π* (1B1) transition for pyrimidine inwater. Hydrogen bonding and first-solvation-shell-specific complexationare included by means of explicit solvent molecules, and solute–solventdispersion is included by using the solvation model with state-specificpolarizability (SMSSP). Geometries of microsolvated clusters weretreated in two different ways, (i) using single liquid-phase global-minimumsolute–solvent clusters containing up to two explicit solventmolecules and (ii) using solute–solvent cluster snapshots derivedfrom molecular dynamics (MD) trajectories. The calculations in waterinvolve using VEM/TDDFT excitation energies and oscillator strengthscomputed over 200 MD-derived solute–solvent clusters and convolutedwith Gaussian functions. We also calculate ground- and excited-statedipole moments for interpretation. We find that inclusion of explicitsolvent molecules generally improves the agreement with experimentand can be recommended as a way to include the effect of hydrogenbonding in solvatochromic shifts. [ABSTRACT FROM AUTHOR]

Isobutanol is a prototype biofuel, and sorting out the mechanism of its combustion is an important objective where theoretical modeling can provide information that is unavailable and not easily obtained by experiment. In the present work the rate constants and branching ratios for the hydrogen abstraction reactions from isobutanol by hydroxyl radical have been calculated using multi-path variational transition-state theory with small-curvature tunneling. We use hybrid degeneracy-corrected vibrational perturbation theory to show that it is critical to consider the anharmonicity difference of high-frequency modes between reactants and transition states. To obtain accurate rate constants, we must apply different scaling factors to the calculated harmonic vibrational frequencies at the reactants and at the transition states. The factors determining the reaction rate constants have been analyzed in detail, including variational effects, tunneling contributions, the effect of multiple reaction paths on transmission coefficients, and anharmonicities of low- and high-frequency vibrational modes. The analysis quantifies the uncertainties in the rate calculations. A key result of the paper is a prediction for the site dependence of hydrogen abstraction from isobutanol by hydroxyl radical. This is very hard to measure experimentally, although it is critical for combustion mechanism modeling. The present prediction differs considerably from previous theoretical work. [ABSTRACT FROM AUTHOR]

This Perspective presents a personal overview of the current status of the theory of chemical kinetics and mechanisms for complex processes. We attempt to assess the status of the field for reactions in the gas phase, at gas-solid interfaces, in liquid solutions, in enzymes, and for protein folding. Some unifying concepts such as potential energy surfaces, free energy, master equations, and reaction coordinates occur in more than one area. We hope this Perspective will be useful for highlighting recent advances and for identifying important areas for future research. [ABSTRACT FROM AUTHOR]

We present vibrational configurationinteraction calculations employingthe Watson Hamiltonian and a multimode expansion. Results for thelowest 36 eigenvalues of the zero total angular momentum rovibrationalspectrum of methane agree with the accurate benchmarks of Wang andCarrington to within a mean unsigned deviation of 0.68, 0.033, and0.014 cm–1for 4-mode, 5-mode, and 6-mode representations,respectively. We note that in the case of the 5-mode results, thisis a factor of 10 better agreement than for 5-mode calculations reportedearlier by Wu, Huang, Carter, and Bowman for the same set of eigenvalues,which indicates that the multimode expansion is even more rapidlyconvergent than previously demonstrated. Our largest calculationsemploy a tiered approach with matrix elements treated using a variable-ordermultimode expansion with orders ranging from 4-mode to 7-mode; strategiesfor assigning matrix elements to particular multimode tiers are discussed.Improvements of 7-mode coupling over 6-mode coupling are small (averaging0.002 cm–1for the first 36 eigenvalues) suggestingthat 7-mode coupling is sufficient to fully converge the results.A number of approximate treatments of the computationally expensivevibrational angular momentum terms are explored. The use of optimizedvibrational quadratures allows rapid integration of the matrix elements,especially the vibrational angular momentum terms, which require significantlyfewer quadrature points than are required to integrate the potential.We assign the lowest 243 states and compare our results to those ofWang and Carrington, who provided assignments for the same set ofstates. Excellent agreement is observed for most states, but our resultsare lower for some of the higher-energy states by as much as 20 cm–1, with the largest deviations being for the stateswith six quanta of excitation in the F2bends, suggesting that the earlier results were not fully convergedwith respect to the basis set. We also provide corrections to severalof the state assignments published previously. [ABSTRACT FROM AUTHOR]

We present global ground-state potential energy surfaces for the quintet and singlet spin states of the O4 system that are suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in electronically adiabatic, spin-conserving O2--O2 collisions. The surfaces are based on MS-CASPT2/maug-cc-pVTZ electronic structure calculations with scaled external correlation. The active space has 16 electrons in 12 orbitals. The calculations cover nine kinds of geometrical arrangements corresponding to dissociative diatom-diatom collisions of O2, geometries corresponding to O3-O, geometries identified by running trajectories, and geometries along linear synchronous transit paths. The global ground-state potential energy surfaces were obtained by a manybody approach with an accurate O--O pairwise interaction and a fit of the many-body interaction to 12 684 electronic structure data points for the singlet and 10 543 electronic structure data points for the quintet. The many-body fit is based on permutationally invariant polynomials in terms of bond-order functions of the six interatomic distances; the bond-order functions are mixed exponential-Gaussian functions. [ABSTRACT FROM AUTHOR]

In the past decade, a number of approaches have been developed to fix the failure of (semi)local density-functional theory (DFT) in describing intermolecular interactions. The performance of several such approaches with respect to highly accurate benchmarks is compared here on a set of separation-dependent interaction energies for ten dimers. Since the benchmarks were unknown before the DFT-based results were collected, this comparison constitutes a blind test of these methods. [ABSTRACT FROM AUTHOR]

Highlights: [•] Simplification of fourfold-way diabatization procedure. [•] Improved algorithm for diabatic states for simulating photochemical dynamics. [•] Complete active space SCF diabatic MOs for excited states with inclusion of dynamical correlation. [Copyright &y& Elsevier]

This work presents a new fragmentmethod, the electrostaticallyembedded many-body expansion of the nonlocal energy (EE-MB-NE), andshows that it, along with thepreviously proposed electrostatically embedded many-body expansionof the correlation energy (EE-MB-CE), produces accurate results forlarge systems at the level of CCSD(T) coupled cluster theory. We primarilystudy water 16-mers, but we also test the EE-MB-CE method on waterhexamers. We analyze the distributions of two-body and three-bodyterms to show why the many-body expansion of the electrostaticallyembedded correlation energy converges faster than the many-body expansionof the entire electrostatically embedded interaction potential. Theaverage magnitude of the dimer contributions to the pairwise additive(PA) term of the correlation energy (which neglects cooperative effects)is only one-half of that of the average dimer contribution to thePA term of the expansion of the total energy; this explains why themean unsigned error (MUE) of the EE-PA-CE approximation is only one-halfof that of the EE-PA approximation. Similarly, the average magnitudeof the trimer contributions to the three-body (3B) term of the EE-3B-CEapproximation is only one-fourth of that of the EE-3B approximation,and the MUE of the EE-3B-CE approximation is one-fourth that of theEE-3B approximation. Finally, we test the efficacy of two- and three-bodydensity functional corrections. One such density functional correctionmethod, the new EE-PA-NE method, with the OLYP or the OHLYP densityfunctional (where the OHLYP functional is the OptX exchange functionalcombined with the LYP correlation functional multiplied by 0.5), hasthe best performance-to-price ratio of any method whose computationalcost scales as the third power of the number of monomers and is competitivein accuracy in the tests presented here with even the electrostaticallyembedded three-body approximation. [ABSTRACT FROM AUTHOR]

The formation ofatmospheric aerosol particles through clusteringof condensable vapors is an important contributor to the overall concentrationof these atmospheric particles. However, the details of the nucleationprocess are not yet well understood and are difficult to probe byexperimental means. Computational chemistry is a powerful tool forgaining insights about the nucleation mechanism. Here, we report accurateelectronic structure calculations of the potential energies of smallclusters made from sulfuric acid, ammonia, and dimethylamine. We alsoassess and validate the accuracy of less expensive methods that mightbe used for the calculation of the binding energies of larger clustersfor atmospheric modeling. The PW6B95-D3 density-functional-plus-molecular-mechanicscalculation with the MG3S basis set stands out as yielding excellentaccuracy while still being affordable for very large clusters. [ABSTRACT FROM AUTHOR]

Abstract: We present benchmark calculations of nine selected points on potential energy surfaces describing proton transfer processes in three model systems, , CH3OH…H+…OH2, and CH3COOH…OH2. The calculated relative energies of these geometries are compared to those calculated by various wave function and density functional methods, including the polarized molecular orbital (PMO) model recently developed in our research group and other semiempirical molecular orbital methods. We found that the SCC-DFTB and PMO methods (the latter available so far only for molecules consisting of only O and H and therefore only for the first of the three model systems) give results that are, on average, within 2kcal/mol of the benchmark results. Other semiempirical molecular orbital methods have mean unsigned errors (MUEs) of 3–8kcal/mol, local density functionals have MUEs in the range 0.7–3.7kcal/mol, and hybrid density functionals have MUEs of only 0.3–1.0kcal/mol, with the best density functional performance obtained by hybrid meta-GGAs, especially M06 and PW6B95. [Copyright &y& Elsevier]

We propose a new formulation of variational transition state theory called multipath variational transition state theory (MP-VTST). We employ this new formulation to calculate the forward and reverse thermal rate constant of the 1,4-hydrogen shift isomerization of the 2-cyclohexylethyl radical in the gas phase. First, we find and optimize all the local-minimum-energy structures of the reaction, product, and transition state. Then, for the lowest-energy transition state structures, we calculate the reaction path by using multiconfiguration Shepard interpolation (MSCI) method to represent the potential energy surface, and, from this representation, we also calculate the ground-state vibrationally adiabatic potential energy curve, the reaction-path curvature vector, and the generalized free energy of activation profile. With this information, the path-averaged generalized transmission coefficients (y) are evaluated. Then, thermal rate constant containing the multiple-structure anharmonicity and torsional anharmonicity effects is calculated using multistructural transition state theory (MS-TST). The final MP-VTST thermal rate constant is obtained by multiplying kMS-TMS-TST by (y). In these calculations, the M06 density functional is utilized to compute the energy, gradient, and Hessian at the Shepard points, and the M06-2X density functional is used to obtain the structures (conformers) of the reactant, product, and the saddle point for computing the multistructural anharmonicity factors. [ABSTRACT FROM AUTHOR]

Distinguishing between the concerted second-order mechanism for β-eliminations and nonconcerted mechanisms with discrete carbanion intermediates is very difficult experimentally, but the ability of quantum chemistry to find stationary points of the free-energy surface in liquid-phase solutions, even for complex reagents, provides a new tool for elucidating such mechanisms. Here we use liquid-phase density functional theory calculations to find transition states and intermediates on the free-energy surfaces of four base-initiated α,β-eliminations of acetoxy and mesyloxy esters and their analogous thioesters. The geometries, free energies, and charge distributions of these structures support a stepwise irreversible first-order elimination from a conjugate base (E1cBI) mechanism with acetoxy ester 3, acetoxy thioester 4, and mesyloxy thioester 6. However, mesyloxy ester 5, which has an excellent nucleofuge and a less-acidic proton, follows a concerted but asynchronous E2 mechanism with an E1cB-like transition state. The anti transition state is more favorable than the syn one, even for the poorer nucleofuge and more-acidic thioesters. The article includes a general scheme for describing liquid-phase reactions in terms of free-energy surfaces. [ABSTRACT FROM AUTHOR]

Abstract: The structures of Pd(PH3)2 and Pt(PH3)2 complexes with ethene and conjugated C n H n +2 systems (n =4, 6, 8, and 10) were studied. Their binding energies were calculated using both wave function theory (WFT) and density functional theory (DFT). Previously it was reported that the binding energy of the alkene to the transition metal does not depend strongly on the size of the conjugated C n H n +2 ligand, but that DFT methods systematically underestimate the binding energy more and more significantly as the size of the conjugated system is increased. Our results show that recently developed density functionals predict the binding energy for these systems much more accurately. New benchmark calculations carried out by the coupled cluster method based on Brueckner orbitals with double excitations and a quasiperturbative treatment of connected triple excitations (BCCD(T)) with a very large basis set agree even better with the DFT predictions than do the previous best estimates. The mean unsigned error in absolute and relative binding energies of the alkene ligands to Pd(PH3)2 is 2.5kcal/mol for the ωB97 and M06 density functionals and 2.9kcal/mol for the M06-L functional. Adding molecular mechanical damped dispersion yields even smaller mean unsigned errors: 1.3kcal/mol for the M06-D functional, 1.5kcal/mol for M06-L-D, and 1.8kcal/mol for B97-D and ωB97X-D. The new functionals also lead to improved accuracy for the analogous Pt complexes. These results show that recently developed density functionals may be very useful for studying catalytic systems involving Pd d10 centers and alkenes. [Copyright &y& Elsevier]

Theoretical studies on the electrode materials in lithium-ion batteries provide information on the structural changes during the charging and discharging processes. In the present study, we tested the M06-L and N12 exchange-correlation functionals on some well-studied lithium-containing materials. These functionals, which have already shown good performance for a variety of databases, outperform the widely used PBE functional for reproducing the experimental structures and averaged intercalation potentials. It is especially noteworthy that the M06-L functional gives voltages as accurate as those provided by the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, but with less computational cost. [ABSTRACT FROM AUTHOR]

We present a new model for predicting branching ratios of chemical reactions when a branching of the reaction path occurs after the dynamical bottleneck, including the case where it occurs after an intermediate. The model is based on combining nonstatistical phase space theory for the direct component of a reaction with variational transition-state theory for an indirect component of reaction. The competition between direct and indirect processes is treated by an extension of the unified statistical model. This new method provides a way to understand the factors that control this kind of chemical reaction and to perform calculations using high-level electronic structure methods for complex systems. The model is based on quantized energy levels of transition states and products, and it involves the same information as required for calculating transition-state rate constants and equilibrium constants plus a phenomenological relaxation time, which was taken from previous work. For the textbook reaction of the hydroboration of propene by BH3 it has recently been inferred that the selectivity can only be understood by consideration of dynamical trajectories. However, the calculated branching fraction of this prototype reaction increases from 2%-3% when calculated under the inappropriate assumption of complete equilibration of the intermediate to from 8%-9% when calculated with the new theory, which requires only limited information about the system and does not involve running trajectories. The calculated result is in reasonable agreement with experiment (∼10%). [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]

Holthausen has recently provided a comprehensive study of density functional theory for calculating the s/d excitation energies of the 3d transition metal cations. This study did not include the effects of scalar relativistic effects, and we show here that the inclusion of scalar relativistic effects significantly alters the conclusions of the study. We find, contrary to the previous study, that local functionals are more accurate for the excitation energies of 3d transition method cations than hybrid functionals. The most accurate functionals, of the 38 tested, are SLYP, PBE, BP86, PBELYP, and PW91. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [ABSTRACT FROM AUTHOR]