Search

We evaluate the effect of electronic decoherence on intersystem crossing in the photodynamics of thioformaldehyde. First, we show that the state-averaged complete-active-space self-consistent field electronic structure calculations with a properly chosen active space of 12 active electrons in 10 active orbitals can predict the potential energy surfaces and the singlet–triplet spin–orbit couplings quite well for CH2S, and we use this method for direct dynamics by coherent switching with decay of mixing (CSDM). We obtain similar dynamical results with CSDM or by adding energy-based decoherence to trajectory surface hopping, with the population of triplet states tending to a small steady-state value over 500 fs. Without decoherence, the state populations calculated by the conventional trajectory surface hopping method or the semiclassical Ehrenfest method gradually increase. This difference shows that decoherence changes the nature of the results not just quantitatively but qualitatively. [ABSTRACT FROM AUTHOR]

The accurate description of ground- and excited-state potential energy surfaces poses a challenge for many electronic structure methods, especially in regions where strong electronic state interaction occurs. Here we introduce a new methodology, state-interaction pair-density functional theory (SI-PDFT), to target molecular systems exhibiting strong interaction of electronic states. SI-PDFT is an extension of multiconfiguration pair-density functional theory in which a set of N electronic states is generated through the diagonalization of an N × N effective Hamiltonian. We demonstrate the accuracy of the method by performing calculations on the ionic–neutral avoided crossing in lithium fluoride and the 1ππ-1πσ* avoided crossing in the H—O bond photodissociation in phenol. We show that SI-PDFT can be a useful tool in the study of photochemistry and nonadiabatic dynamics. [ABSTRACT FROM AUTHOR]

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]

With the goal of accelerating the design and discovery of metal–organic frameworks (MOFs) for electronic, optoelectronic, and energy storage applications, we present a dataset of predicted electronic structure properties for thousands of MOFs carried out using multiple density functional approximations. Compared to more accurate hybrid functionals, we find that the widely used PBE generalized gradient approximation (GGA) functional severely underpredicts MOF band gaps in a largely systematic manner for semi-conductors and insulators without magnetic character. However, an even larger and less predictable disparity in the band gap prediction is present for MOFs with open-shell 3d transition metal cations. With regards to partial atomic charges, we find that different density functional approximations predict similar charges overall, although hybrid functionals tend to shift electron density away from the metal centers and onto the ligand environments compared to the GGA point of reference. Much more significant differences in partial atomic charges are observed when comparing different charge partitioning schemes. We conclude by using the dataset of computed MOF properties to train machine-learning models that can rapidly predict MOF band gaps for all four density functional approximations considered in this work, paving the way for future high-throughput screening studies. To encourage exploration and reuse of the theoretical calculations presented in this work, the curated data is made publicly available via an interactive and user-friendly web application on the Materials Project. [ABSTRACT FROM AUTHOR]

The photodissociation of thioanisole is very interesting because the experiments of Lim and Kim provide evidence for mode-specific effects on the product distribution. They showed that, with a specific S-CH3 stretching mode being excited as the reagent is excited to the S1 electronic state, there is a sharp increase in the proportion of the ground-state product to the excited-state product. In the present work, we report 78 011 full-dimensional semiclassical multi-state trajectories of the photodissociation process using the coherent switching with decay of mixing dynamics method. The potential surfaces and couplings are based on electronic structure calculations that include dynamic correlation through second order perturbation theory. We report results for four sets of initial conditions, one corresponding roughly to 0-0 excitation and three corresponding to exciting one vibrational mode, to look for modespecific effects. The simulations show no significant mode-specific effect on the product energy distributions, but they do showan effect on the distribution of minimum-energy gaps in the trajectories and on the lifetime for dissociation. In particular, excitation of the S-CH3 stretching mode leads to trajectories passing closer to the S1-S2 conical intersection and to shorter lifetimes. This provides a possible explanation of why experimental results are different for excitation of this vibration. [ABSTRACT FROM AUTHOR]

Analytic potential energy surfaces (PESs) and state couplings of the ground and two lowest singlet excited states of thioanisole (C6H5SCH3) are constructed in a diabatic representation based on electronic structure calculations including dynamic correlation. They cover all 42 internal degrees of freedom and a wide range of geometries including the Franck-Condon region and the reaction valley along the breaking S-CH3 bond with the full ranges of the torsion angles. The parameters in the PESs and couplings are fitted to the results of smooth diabatic electronic structure calculations including dynamic electron correlation by the extended multi-configurational quasi-degenerate perturbation theory method for the adiabatic state energies followed by diabatization by the fourfold way. The fit is accomplished by the anchor points reactive potential method with two reactive coordinates and 40 nonreactive degrees of freedom, where the anchor-point force fields are obtained with a locally modified version of the QuickFF package. The PESs and couplings are suitable for study of the topography of the trilayer potential energy landscape and for electronically nonadiabatic molecular dynamics simulations of the photodissociation of the S-CH3 bond. [ABSTRACT FROM AUTHOR]

This article presents a perspective on Kohn-Sham density functional theory (KS-DFT) for electronic structure calculations in chemical physics. This theory is in widespread use for applications to both molecules and solids. We pay special attention to several aspects where there are both concerns and progress toward solutions. These include: 1. The treatment of open-shell and inherently multiconfigurational systems (the latter are often called multireference systems and are variously classified as having strong correlation, near-degeneracy correlation, or high static correlation; KS-DFT must treat these systems with broken-symmetry determinants). 2. The treatment of noncovalent interactions. 3. The choice between developing new functionals by parametrization, by theoretical constraints, or by a combination. 4. The ingredients of the exchange-correlation functionals used by KS-DFT, including spin densities, the magnitudes of their gradients, spin-specific kinetic energy densities, nonlocal exchange (Hartree-Fock exchange), nonlocal correlation, and subshell-dependent corrections (DFT+U). 5. The quest for a universal functional, where we summarize some of the success of the latest Minnesota functionals, namely MN15-L and MN15, which were obtained by optimization against diverse databases. 6. Time-dependent density functional theory, which is an extension of DFT to treat time-dependent problems and excited states. The review is a snapshot of a rapidly moving field, and--like Marcel Duchamp--we hope to convey progress in a stimulating way. [ABSTRACT FROM AUTHOR]

We recently proposed the dipole-quadrupole (DQ) method for transforming adiabatic electronic states to diabatic states by using matrix elements of the dipole and quadrupole operators, and we applied the method to 3-state diabatizations of LiH and phenol. Here we extend the method to also include the electrostatic potential, and we call the resulting method the DQΦ method, which denotes the dipoleâ€"quadrupoleâ€"electrostatic-potential diabatization method. The electrostatic potential provides extra flexibility, and the goal of the present work is to test and illustrate the robustness of the methods for producing diabatic potential energy curves that tend to the adiabatic curves away from crossings and avoided crossings and are smooth in regions of crossings and avoided crossings. We illustrate the generality of the methods by an application to LiH with four states and by two-state diabatizations of HCl, (H2)2, O3, and the reaction Li + HF â?' LiF + H. We find thatÂ--if enough states are includedÂ--the DQ method does not have a significant dependence on the parameter weighting the quadrupole moment, and a geometryindependent value of 10 a0-2 is adequate in all cases tested. We also find that the addition of the electrostatic potential improves the diabatic potentials in some cases and provides an additional property useful for increasing the generality of the method for diabatization. [ABSTRACT FROM AUTHOR]

Diabatization is a procedure that transforms multiple adiabatic electronic states to a new representation in which the potential energy surfaces and the couplings between states due to the electronic Hamiltonian operator are smooth, and the couplings due to nuclear momentum are negligible. In this work, we propose a simple and general diabatization strategy, called model space diabatization, that is applicable to multi-configuration quasidegenerate perturbation theory (MC-QDPT) or its extended version (XMC-QDPT). An advantage over previous diabatization schemes is that dynamical correlation calculations are based on standard post-multi-configurational self-consistent field (MCSCF) multi-state methods even though the diabatization is based on state-averaged MCSCF results. The strategy is illustrated here by applications to LiH, LiF, and thioanisole, with the fourfold-way diabatization and XMC-QDPT, and the results illustrate its validity. [ABSTRACT FROM AUTHOR]

The diatomic molecule Fe2 was investigated using restricted active space second-order perturbation theory (RASPT2). This molecule is very challenging to study computationally because predictions about the ground state and excited states depend sensitively on the choice of the quantum chemical method. For Fe2 we show that one needs to go beyond a full-valence active space in order to achieve even qualitative agreement with experiment for the dissociation energy, and we also obtain a smooth ground-state potential curve. In addition we report the first multireference study of Fe2+, for which we predict an 8σu- ground state, which was not predicted by previous computational studies. By using an active space large enough to remove the most serious deficiencies of previous theoretical work and by explicitly investigating the interpretations of previous experimental results, this study elucidates previous difficulties and provides - for the first time - a qualitatively correct treatment of Fe2, Fe2+, and Fe2-. Moreover, this study represents a record in terms of the number or active electrons and active orbitals in the active space, namely 16 electrons in 28 orbitals. Conventional CASPT2 calculations can be performed with at most 16 electrons in 16 orbitals. We were able to overcome this limit by using the RASPT2 formalism. [ABSTRACT FROM AUTHOR]

Faced with the contradictory results of two recent experimental studies [Jara‐Toro et al., Angew. Chem. Int. Ed. 2017, 56, 2166 and Chao et al., Angew. Chem. Int. Ed. 2019, 58, 5013] of the possible catalytic effect of water vapor on CH3OH + OH reaction, we report calculations that corroborate the conclusion made by Chao et al. and extend the rate constant evaluation down to 200 K. The rate constants of the CH3OH + OH reaction catalyzed by a water molecule are computed as functions of temperature and relative humidity using high‐level electronic structure and kinetics calculations. The Wuhan–Minnesota Scaling (WMS) method is used to provide accurate energetics to benchmark a density functional for direct dynamics. Both high‐frequency and low‐frequency anharmonicities are included. Variational and tunneling effects are treated by canonical variational transition state theory with multidimensional small‐curvature tunneling. And, most significantly, we include multistructural effects in the rate constant calculations. Our calculations show that the catalytic effect of water vapor is not observable at 200–400 K. [ABSTRACT FROM AUTHOR]

Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the highpressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C2H2→C2H2Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimental results. The chlorovinyl (C2H2Cl) adduct has two isomers that are equilibrium structures of a double-well C≡C-H bending potential. Two procedures are used to calculate the vibrational partition function of chlorovinyl; one treats the two isomers separately and the other solves the anharmonic energy levels of the double well.We use these results to calculate the standard-state free energy and equilibrium constant of the reaction. [ABSTRACT FROM AUTHOR]

We calculate the standard state entropy, heat capacity, enthalpy, and Gibbs free energy for 13 radicals important for the combustion chemistry of biofuels. These thermochemical quantities are calculated from recently proposed methods for calculating partition functions of complex molecules by taking into account their multiple conformational structures and torsional anharmonicity. The radicals considered in this study are those obtained by hydrogen abstraction from 1-butanol, 2-methyl-1-propanol, and butanal. Electronic structure calculations for all conformers of the radicals were carried out using both density functional theory and explicitly correlated coupled cluster theory with quasipertubative inclusion of connected triple excitations. The heat capacity and entropy results are compared with sparsely available group additivity data, and trends in enthalpy and free energy as a function of radical center are discussed for the isomeric radicals. [ABSTRACT FROM AUTHOR]

As electronic structure methods are being used to obtain quantitatively accurate reaction energies and barrier heights for increasingly larger systems, the choice of an efficient basis set is becoming more critical. The optimum strategy for achieving basis set convergence can depend on the way that electron correlation is treated and can take advantage of flexibility in the order in which basis functions are added. Here we study several approaches for estimating accurate reaction energies and barrier heights from post-Hartree-Fock electronic structure calculations. First and second, we evaluate methods of estimating the basis set limit of second order Mo\ller-Plesset perturbation theory and of coupled cluster theory with single and double excitations and a quasiperturbative treatment of connected triple excitations by using explicitly correlated basis functions (in the F12a implementation) along with valence, polarization, and diffuse one-electron basis functions. Third, we test the scheme of adding a higher-order correction to MP2 results (sometimes called MP2/CBS + ΔCCSD(T)). Finally, we evaluate the basis set requirements of these methods in light of comparisons to Weizmann-3.2, Weizmann-4, and CCSDT(2)Q/CBS+CV+R results. [ABSTRACT FROM AUTHOR]

The purpose of the present investigation is to calculate partition functions and thermodynamic quantities, viz., entropy, enthalpy, heat capacity, and Gibbs free energies, for 1-butanol, 2-methyl-1-propanol, and butanal in the vapor phase. We employed the multi-structural (MS) anharmonicity method and electronic structure calculations including both explicitly correlated coupled cluster theory and density functional theory. The calculations are performed using all structures for each molecule and employing both the local harmonic approximation (MS-LH) and the inclusion of torsional anharmonicity (MS-T). The results obtained from the MS-T calculations are in excellent agreement with experimental data taken from the Thermodynamics Research Center data series and the CRC Handbook of Chemistry and Physics, where available. They are also compared with Benson's empirical group additivity values, where available; in most cases, the present results are more accurate than the group additivity values. In other cases, where experimental data (but not group additivity values) are available, we also obtain good agreement with experiment. This validates the accuracy of the electronic structure calculations when combined with the MS-T method for estimating the thermodynamic properties of systems with multiple torsions, and it increases our confidence in the predictions made with this method for molecules and temperatures where experimental or empirical data are not available. [ABSTRACT FROM AUTHOR]

We extend our recent SOGGA11 approximation to the exchange-correlation functional to include a percentage of Hartree-Fock exchange. The new functional, called SOGGA11-X, has better overall performance for a broad chemical database than any previously available global hybrid generalized gradient approximation, and in addition it satisfies an extra physical constraint in that it is correct to second order in the density-gradient. [ABSTRACT FROM AUTHOR]

This paper describes and illustrates a way to construct multidimensional representations of reactive potential energy surfaces (PESs) by a multiconfiguration Shepard interpolation (MCSI) method based only on gradient information, that is, without using any Hessian information from electronic structure calculations. MCSI, which is called multiconfiguration molecular mechanics (MCMM) in previous articles, is a semiautomated method designed for constructing full-dimensional PESs for subsequent dynamics calculations (classical trajectories, full quantum dynamics, or variational transition state theory with multidimensional tunneling). The MCSI method is based on Shepard interpolation of Taylor series expansions of the coupling term of a 2×2 electronically diabatic Hamiltonian matrix with the diagonal elements representing nonreactive analytical PESs for reactants and products. In contrast to the previously developed method, these expansions are truncated in the present version at the first order, and, therefore, no input of electronic structure Hessians is required. The accuracy of the interpolated energies is evaluated for two test reactions, namely, the reaction OH+H2→H2O+H and the hydrogen atom abstraction from a model of α-tocopherol by methyl radical. The latter reaction involves 38 atoms and a 108-dimensional PES. The mean unsigned errors averaged over a wide range of representative nuclear configurations (corresponding to an energy range of 19.5 kcal/mol in the former case and 32 kcal/mol in the latter) are found to be within 1 kcal/mol for both reactions, based on 13 gradients in one case and 11 in the other. The gradient-based MCMM method can be applied for efficient representations of multidimensional PESs in cases where analytical electronic structure Hessians are too expensive or unavailable, and it provides new opportunities to employ high-level electronic structure calculations for dynamics at an affordable cost. [ABSTRACT FROM AUTHOR]

Multiconfiguration molecular mechanics (MCMM) was previously applied to calculate potential energies, gradients, and Hessians along a reaction path and in the large-curvature tunneling swath, and it was shown that one could calculate variational transition state theory rate constants with optimized multidimensional tunneling without requiring more than a few electronic structure Hessians. It was also used for molecular dynamics simulations of liquid-phase potentials of mean force as functions of a reaction coordinate. In the present article we present some improvements to the formalism and also show that with these improvements we can use the method for the harder problem of trajectory calculations on gas-phase bimolecular reactive collisions. In particular, we apply the MCMM algorithm to the model reaction OH+H2→H2O+H, for which we construct the global full-dimensional interpolated potential energy surfaces with various numbers of electronic structure Hessians and various molecular mechanics force fields, and we assess the quality of these fits by quasiclassical trajectory calculations. We demonstrate that chemical accuracy (1–2 kcal/mol) can be reached for a MCMM potential in dynamically important regions with a fairly small number of electronic structure Hessians. We also discuss the origins of the errors in the interpolated energies and a possible way to improve the accuracy. [ABSTRACT FROM AUTHOR]

A previous article proposed an electronic structure-based polarizable potential, called the explicit polarization (X-POL) potential, to treat many-body polarization and charge delocalization effects in polypeptides. Here, we present a variational version of the X-POL potential, in which the wave function of the entire molecular system is variationally optimized to yield the minimum total electronic energy. This allows the calculation of analytic gradients, a necessity for efficient molecular dynamics simulations. In this paper, the detailed derivations of the Fock matrix and analytic force are presented and discussed. The calculations involve a double self-consistent-field procedure in which the wave function of each fragment is self-consistently optimized in the presence of other fragments, and in addition the polarization of the entire system is self-consistently optimized. The variational X-POL potential has been implemented in the Chemistry at Harvard Molecular Mechanics (CHARMM) package and tested successfully for small model compounds. [ABSTRACT FROM AUTHOR]

The performance of the M06 family of exchange-correlation potentials for describing the electronic structure and the Heisenberg magnetic coupling constant (J) is investigated using a set of representative open-shell systems involving two unpaired electrons. The set of molecular systems studied has well defined structures, and their magnetic coupling values are known experimentally. As a general trend, the M06 functional is about equally as accurate as B3LYP or PBE0. The performance of local functionals is important because of their economy and convenience for large-scale calculations; we find that M06-L local functional of the M06 family largely improves over the local spin density approximation and the generalized gradient approximation. [ABSTRACT FROM AUTHOR]

Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and co-workers [J. Chem. Phys. 95, 3848 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH2C(O)Cl was prepared in its ground electronic state (S0) and excited with a laser at 248 nm to its first excited singlet state (S1). The two main ensuing photoreactions are the ruptures of the C–Cl bond and of the C–Br bond. A nonadiabatic model was proposed in which the C–Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S1 and S2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C–Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S0 and S1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C–Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C–Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations. [ABSTRACT FROM AUTHOR]

Presents high-level ab initio electronic structure calculations for the ground and first excited state of the NaH[sub 2] system. Inclusion of full configuration interaction theory and second-order Moller-Plesset perturbation theory; Comparison of the matrix potential and its eigenvalues to previous literature.

Abstract Electronically nonadiabatic photodissociation can be investigated in detail by experiments. This provides an opportunity to validate the current theory of electronically nonadiabatic processes, but is great challenge because the time scale of photodissociation processes can be long compared to the current capability for accurate direct dynamics simulations. To circumvent this difficulty, we have been using analytic diabatic potential energy matrices, such that the simulation time scale can be extended to the nanosecond region without neglecting the effects of external electron correlation. In previous work, we have developed full-dimensional three-state potential energy matrices for thioanisole photodissociation. In the current work, we extend this work in two main ways: (i) we improve the treatment of the initial torsional potential, and (ii) we shift the potential energy surfaces to investigate the quantitative effect on the dissociation lifetimes and product branching ratios of changing the energy and location of the S 1 –S 2 conical intersection. [ABSTRACT FROM AUTHOR]

We define a general set of effective solvent coordinates for reactions in solution that should be especially well suited for polar solvents. No specific functional form is assumed for the variation of charges with motions along or orthogonal to the reaction coordinate, and as a consequence the formalism allows the treatment of arbitrary reaction types using electronic structure calculations for the solute charge distribution. A critical assumption of the theory is that the nonequilibrium polarization field may be represented in terms of the adiabatic polarization fields that arise for linear geometric displacements. The results are combined with the Cartesian-coordinate canonical variational transition state theory presented previously to provide a general method for including both equilibrium and nonequilibrium solvation effects of a nondissipative, continuum-dielectric solvent in practical calculations of rate constants. [ABSTRACT FROM AUTHOR]

A new potential energy surface is presented for the reaction F+H2→HF+H. The regions of the surface corresponding to collinear and bent geometries in the F–H–H and H–F–H barrier regions are based on scaled external correlation (SEC) electronic structure calculations, and the F–H···H exit channel region is based on the previously developed surface No. 5. The functional form of the new surface includes dispersion forces by a double many-body expansion (DMBE), and the surface was adjusted so that the van der Waals well in the F···H–H region agrees with available experimental predictions. We have calculated stationary point properties for the new surface as well as product–valley barrier maxima of vibrationally adiabatic potential curves for F+H2→HF(v’=3)+H,F+HD→HF(v’=3)+D, and F+D2→DF(v’=4)+D. The new surface should prove useful for studying the effect on dynamics of a low, early barrier with a wide, flat bend potential, as indicated by the best available electronic structure calculations. [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 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 electronic spectrum of permanganate ions contains various highly multiconfigurational ligand-to-metal charge transfer states and is notorious for being one of the most challenging systems to be treated by quantum-chemical methods. Here we studied the lowest nine vertical excitation energies using restricted active space second-order perturbation theory (RASPT2) and multiconfiguration pair-density functional theory (MC-PDFT) to test and compare these two theories in computing such a challenging spectrum. The results are compared to literature data, including time-dependent density functional theory, completely renormalized equation-of-motion couple-cluster theory with single and double excitations, symmetry-adapted-cluster configuration interaction, and experimental spectra in the gas phase and solution. Our results show that MC-PDFT accurately predicts the spectrum at a significantly reduced cost as compared to RASPT2. [ABSTRACT FROM AUTHOR]

The electronic spectrum of permanganate ions contains various highly multiconfigurational ligand-to-metal charge transfer states and is notorious for being one of the most challenging systems to be treated by quantum-chemical methods. Here we studied the lowest nine vertical excitation energies using restricted active space second-order perturbation theory (RASPT2) and multiconfiguration pair-density functional theory (MC-PDFT) to test and compare these two theories in computing such a challenging spectrum. The results are compared to literature data, including time-dependent density functional theory, completely renormalized equation-of-motion couple-cluster theory with single and double excitations, symmetry-adapted-cluster configuration interaction, and experimental spectra in the gas phase and solution. Our results show that MC-PDFT accurately predicts the spectrum at a significantly reduced cost as compared to RASPT2. [ABSTRACT FROM AUTHOR]

Cyclobutanetetrone, (CO)4, has a triplet ground state. Here we predict, based on electronic structure calculations, that the B2N2O4 molecule also has a triplet ground state and is therefore paramagnetic; the structure is an analogue of (CO)4 in which the carbon ring is replaced by a (BN)2 ring. Similar to (CO)4, the triplet ground-state structure of B2N2O4 is also thermodynamically unstable. Besides analysis of the molecular orbitals, we found that the partial atomic charges are good indicators for predicting magnetic ground states. [ABSTRACT FROM AUTHOR]

Beginning with the MIDI! basis set (also called MIDIX), we introduce the MIDIX+, MIDIY, and MIDIY+ basis sets. Using correlated ab initio and hybrid density functional theory, we compare their performance to that of several existing basis sets for electronic structure calculations. The new basis sets are tested with databases of 358 energies of reactions, 44 barrier heights, 31 electron affinities, 18 geometries, and 29 dipole moments. The MIDI!, MIDIX+, MIDIY, and MIDIY+ basis sets are shown to be cost-efficient methods for calculating relative energies, geometries, and dipole moments. The MIDIX+ basis is shown to be particularly efficient for calculating electron affinities of large molecules. [ABSTRACT FROM AUTHOR]

Reports on large-scale electronic structure calculations of the potential energy surface for the F + H[sub2]→HF + H reaction, with emphasis on the saddle point region. Steps involved in the energy calculation; Bend potential basis as a function of bond angle in degrees.

We report electronically nonadiabatic dynamics calculations including spin-orbit coupling for the photodissociation of CH2ClBr to yield Cl(2P3/2), Cl(2P1/2), Br(2P3/2), and Br(2P1/2). The potential energy is a 24 × 24 matrix (divided up here into four 6 × 6 blocks in a first approximation to the problem), in a spin-coupled fully diabatic representation obtained by combining the spin-free fourfold way with single-center spin-orbit coupling constants. The spin-free calculations are carried out by multiconfiguration quasidegenerate perturbation theory, and the fully diabatic potentials including spin-orbit coupling are fit to a matrix reactive force field. The dynamics are carried out by the coherent switches with decay of mixing method in the diabatic representation. The results show qualitative agreement with experiment. [ABSTRACT FROM AUTHOR]

Presents corrections to the article, entitled `Infinite basis limits in electronic structure theory' published in the volume 111 issue of the `Journal of Chemical Physics'.

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]

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 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]

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.

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]

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]

Metal nanoparticles are important in several emerging technologies, but their size-selected thermodynamic properties are hard to obtain from experiment. We have characterized the energetic and structural properties of unsupported neutral Aln (2 ⩽ n ⩽ 65) particles (clusters and nanoparticles) via molecular dynamics quenching simulations with a recently validated many-body analytic potential. For each particle size (n), the global minimum-energy structure, the distribution of the local energy minima, and the finite-temperature thermodynamics have been calculated, the latter by evaluating ∼100 000 rovibrational partition functions for the low-energy isomers of the various particles. This analysis demonstrates that the dominant structures of clusters and nanoparticles depend on temperature as well as particle size and that one must consider statistical mechanics as well as electronic structure in determining the dominant structures, stabilities, and properties of nanoparticles. As a particularly dramatic example, although the electronic magic numbers of Aln are n = 13, 19, 23, 38, and 55 when thermal energy is neglected, the n = 38 magic number is found to become unstable relative to its neighbors (n = 37 and 39) at temperatures of 500 K and above due to vibrational energy and entropy effects. Furthermore, an energy-landscape analysis based on the probability of finding an isomer demonstrates that for many particle sizes, the global-minimum-energy structure on the potential energy surface is not the dominant structure at moderate temperatures, and other low-energy isomers may be dominant at temperatures as low as room temperature. For example, the four lowest-energy structures account for less than 50% of the population for n = 17, 31, 33, 34, 36, 37, 39, 41-43, 50, 56, 58, and 63-65 at 300 K and for n = 10, 11, and 17-65 at 1500 K. At 1500 K, even the 64 lowest-energy structures account for less than half the population for n = 23, 27-55, and 57-65. The increased importance of higher-energy structures at finite temperatures has important implications for understanding the size-selective reactivity and catalytic activity of metal nanoparticles. The isomeric energy (Elso), which is the difference between the thermal average energy of the particle and that of the corresponding global minimum structure in the ground electronic state, is introduced as an indicator of how well the thermochemical properties of a multi-isomer particle can be represented by those of the global minimum structure. Particularly low values for Al12, Al13, Al19, Al48, Al53, Al54, and Al56 have been found in a wide temperature range. [ABSTRACT FROM AUTHOR]

The article describes the development of a multidimensional potential energy surface for the CH5 system. According to the authors, the quality and accuracy of the kinetic and dynamic description of a chemical reaction is strongly dependent on the quality of the potential-energy surface. They also explain methods for constructing potential-energy surfaces, electronic structure theory surfaces and kinetic isotope effects.

We report electronic, vibrational, and magnetic properties, together with their structural dependences, for the metal-organic framework Fe2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) and its derivatives, Fe2(O)2(dobdc) and Fe2(OH)2(dobdc)--species arising in the previously proposed mechanism for the oxidation of ethane to ethanol using N2O as an oxidant. Magnetic susceptibility measurements reported for Fe2(dobdc) in an earlier study and reported in the current study for FeII0.26[FeIII(OH)]1.74(dobdc)(DMF)0.15(THF)0.22, which is more simply referred to as Fe2(OH)2(dobdc), were used to confirm the computational results. Theory was also compared to experiment for infrared spectra and powder X-ray diffraction structures. Structural and magnetic properties were computed by using Kohn-Sham density functional theory both with periodic boundary conditions and with cluster models. In addition, we studied the effects of different treatments of the exchange interactions on the magnetic coupling parameters by comparing several approaches to the exchange-correlation functional: generalized gradient approximation (GGA), GGA with empirical Coulomb and exchange integrals for 3d electrons (GGA+U), nonseparable gradient approximation (NGA) with empirical Coulomb and exchange integrals for 3d electrons (NGA+U), hybrid GGA, meta-GGA, and hybrid meta-GGA. We found the coupling between the metal centers along a chain to be ferromagnetic in the case of Fe2(dobdc) and antiferromagnetic in the cases of Fe2(O)2(dobdc) and Fe2(OH)2(dobdc). The shift in magnetic coupling behavior correlates with the changing electronic structure of the framework, which derives from both structural and electronic changes that occur upon metal oxidation and addition of the charge-balancing oxo and hydroxo ligands. [ABSTRACT FROM AUTHOR]

The explicit polarization (X-Pol) method is a fragment-basedquantummechanical model, in which a macromolecular system or other largeor complex system in solution is partitioned into monomeric fragments.The present study extends the original X-Pol method, where all fragmentsare treated using the same electronic structure theory, to multilevelrepresentations, called multilevel X-Pol, in which different electronicstructure methods are used to describe different fragments. The multilevelX-Pol method has been implemented into a locally modified versionof Gaussian 09. A key ingredient that is used to couple interfragmentelectrostatic interactions at different levels of theory is the useof the response density for the post-self-consistent-field energy.(The response density is also called the generalized density.) Themethod is useful for treating fragments in a small region of the systemsuch as a solute molecule or the substrate and amino acids in theactive site of an enzyme with a high-level theory, and the fragmentsin the rest of the system by a lower-level and computationally moreefficient method. Multilevel X-Pol is illustrated here by applicationsto hydrogen bonding complexes in which one fragment is treated withthe hybrid M06 density functional, Møller–Plesset perturbationtheory, or coupled cluster theory, and the other fragments are treatedby Hartree–Fock theory or the B3LYP or M06 hybrid density functionals. [ABSTRACT FROM AUTHOR]