Electronic Energy Calculations Research Articles

Indium(0)‐Mediated C–S/O Cross‐Coupling Approach Towards the Regioselective Alkylation of α‐Enolic Esters/Dithioesters: A Mechanistic Insight

AbstractWe have reported an indium(0)‐mediated C–S/O cross‐coupling approach that leads to the highly regioselective alkylation of α‐enolic acetate/dithioacetate systems. This hetero cross‐coupling reaction does not require additional co‐catalyst or promoter, and the in situ generated organoindium species promotes the reaction by acting as the coupling partner of the α‐enolic acetate/dithioacetate substrates. The excellent selectivity for C‐, S‐, or O‐alkylation is solely dependent on the nucleophilic behavior of α‐enolic acetate/dithioacetate systems. These results were further supported by DFT calculations of the relative electronic energies of the substrates and products as well as the activation barriers of the respective transformations.

Erratum: “Performance of recent and high-performance approximate density functionals for time-dependent density functional theory calculations of valence and Rydberg electronic transition energies” [J. Chem. Phys. 137, 244104 (2012)

Erratum: “Performance of recent and high-performance approximate density functionals for time-dependent density functional theory calculations of valence and Rydberg electronic transition energies” [J. Chem. Phys. 137, 244104 (2012)

Rate Constant Calculations of H-Atom Abstraction Reactions from Ethers by HȮ2 Radicals

In this work, we detail hydrogen atom abstraction reactions from six ethers by the hydroperoxyl radical, including dimethyl ether, ethyl methyl ether, propyl methyl ether, isopropyl methyl ether, butyl methyl ether, and isobutyl methyl ether, in order to test the effect of the functional group on the rate constant calculations. The Møller-Plesset (MP2) method with the 6-311G(d,p) basis set has been employed in the geometry optimizations and frequency calculations of all of the species involved in the above reaction systems. The connections between each transition state and the corresponding local minima have been determined by intrinsic reaction coordinate calculations. Energies are reported at the CCSD(T)/cc-pVTZ level of theory and include the zero-point energy corrections. As a benchmark in the electronic energy calculations, the CCSD(T)/CBS extrapolation was used for the reactions of dimethyl ether + HȮ2 radicals. A systematic calculation of the high-pressure limit rate constants has been performed using conventional transition-state theory, including asymmetric Eckart tunneling corrections, in the temperature range of 500-2000 K. The one dimensional hindrance potentials obtained at MP2/6-311G(d,p) for the reactants and transition states have been used to describe the low frequency torsional modes. Herein, we report the calculated individual, average, and total rate constants. A branching ratio analysis for every reaction site has also been performed.

A corrected-linear response formalism for the calculation of electronic excitation energies of solvated molecules with the CCSD-PCM method

In this work, we present an efficient approximation to compute excitation energies in solution when coupled cluster (CC) methods are combined with a polarizable solvation model. Two formalisms exist to compute excited state energies with polarizable solvation models: state-specific (SS) and linear-response (LR). The former more accurately describes the solute–solvent polarization in the excited state, but is computationally intensive. The LR formalism is efficient, but lacks proper relaxation effects. An approximate method, called corrected-LR (cLR), was originally formulated in the context of time-dependent density functional theory and the polarizable continuum model of solvation (PCM), and was shown to be able to recover most of the relaxation contributions of the SS formalism at a cost similar to LR. We have expanded the cLR idea to CC theory, and introduced an extra approximation that further reduces the computational effort with negligible loss of accuracy. The test cases reported in this contribution clearly show that the cLR-CC-PCM method is able to estimate transition energies in very close agreement with the SS formalism at a cost that is similar (in fact, slightly smaller) than the LR formalism. The average SS–LR difference is of the order of 0.10–0.20eV for nonequilibrium calculations, and 0.30–0.55eV for equilibrium calculations. The SS–cLR average difference is, on the other hand, 0.01–0.02eV for nonequilibrium calculations, and 0.05–0.15eV for equilibrium calculations. Therefore, the cLR approach is a promising alternative for computing excited state energies in solution with computationally intensive CC methods.

Theoretical and Kinetic Study of the Hydrogen Atom Abstraction Reactions of Esters with HȮ2 Radicals

This work details an ab initio and chemical kinetic study of the hydrogen atom abstraction reactions by the hydroperoxyl radical (HȮ2) on the following esters: methyl ethanoate, methyl propanoate, methyl butanoate, methyl pentanoate, methyl isobutyrate, ethyl ethanoate, propyl ethanoate, and isopropyl ethanoate. Geometry optimizations and frequency calculations of all of the species involved, as well as the hindrance potential descriptions for reactants and transition states, have been performed with the Møller-Plesset (MP2) method using the 6-311G(d,p) basis set. A validation of all of the connections between transition states and local minima was performed by intrinsic reaction coordinate calculations. Electronic energies for all of the species are reported at the CCSD(T)/cc-pVTZ level of theory in kcal mol(-1) with the zero-point energy corrections. The CCSD(T)/CBS (extrapolated from CCSD(T)/cc-pVXZ, in which X = D, T, Q) was used for the reactions of methyl ethanoate + HȮ2 radicals as a benchmark in the electronic energy calculations. High-pressure limit rate constants, in the temperature range 500-2000 K, have been calculated for all of the reaction channels using conventional transition state theory with asymmetric Eckart tunneling corrections. The 1-D hindered rotor approximation has been used for the low frequency torsional modes in both reactants and transition states. The calculated individual and total rate constants are reported for all of the reaction channels in each reaction system. A branching ratio analysis for each reaction site has also been investigated for all of the esters studied in this work.

Uquantchem: A versatile and easy to use quantum chemistry computational software

In this paper we present the Uppsala Quantum Chemistry package (UQUANTCHEM), a new and versatile computational platform with capabilities ranging from simple Hartree–Fock calculations to state of the art First principles Extended Lagrangian Born–Oppenheimer Molecular Dynamics (XL-BOMD) and diffusion quantum Monte Carlo (DMC). The UQUANTCHEM package is distributed under the general public license and can be directly downloaded from the code web-site (http://www.anst.uu.se/pesou087/DOWNLOAD-UQUANTCHEM/DOWNLOAD-UQUANTCHEM/DOWNLOAD-SITE-UQUANTCHEM.html) [1]. Together with a presentation of the different capabilities of the uquantchem code and a more technical discussion on how these capabilities have been implemented, a presentation of the user-friendly aspect of the package on the basis of the large number of default settings will also be presented. Furthermore, since the code has been parallelized within the framework of the message passing interface (MPI), the timing of some benchmark calculations are reported to illustrate how the code scales with the number of computational nodes for different levels of chemical theory. Program summaryProgram title: UquantchemCatalogue identifier: AEQY_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEQY_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License version 3No. of lines in distributed program, including test data, etc.: 2082722No. of bytes in distributed program, including test data, etc.: 15501085Distribution format: tar.gzProgramming language: Fortran90.Computer: The program should work on any system with a F90 compiler. The code has been tested with the Intel and gfortran compilers.Operating system: Unix/Linux.Has the code been vectorized or parallelized?: The distribution file contains both a serial and a parallel version of the program. Number of processors used, 2–2000.RAM: 2 GB for molecules consisting of <10 atoms.Classification: 16.10.External routines: The Lapack and Blas libraries are required but are included in the distribution file. MPI is required for the parallel version.Nature of problem:Electronic structure, total energy and force calculation of molecules.Solution method:Basis-set expansion in terms of contracted Gaussian functions is used to solve the Hartree–Fock or the Kohn–Sham equations self consistently.Running time:From a couple of seconds up to several days depending on the size of the molecule, the level of theory used and the number of processors used.

Stability, elastic and electronic properties of the Rh–Zr compounds from first-principles calculations

A systematic investigation on structural, elastic and electronic properties of Rh–Zr intermetallic compounds is conducted using first-principles electronic structure total energy calculations. The equilibrium lattice parameters, enthalpies of formation (Efor), cohesive energies (Ecoh) and elastic constants are presented. Of the eleven considered candidate structures, Rh4Zr3 is most stable with the lowest Efor. The two orthogonal-type, relative to the CsCl-type, are the competing ground-state structures of RhZr. The result is in agreement with the experimental reports in the literature. The analysis of Efor and mechanical stability excludes the presence of Rh2Zr and RhZr4 at low temperature mentioned by .Curtarolo et al. [Calphad 29, 163 (2005)]. It is found that the bulk modulus B increases monotonously with Rh concentration, whereas all other quantities (shear modulus G, Young's modulus E, Poisson's ratio σ and ductility measured by B/G) show nonmonotonic variation. RhZr2 exhibits the smallest shear/Young's modulus, the largest Poisson's ratio and ductility. Our results also indicate that all the Rh–Zr compounds considered are ductile. Furthermore, the detailed electronic structure analysis is implemented to understand the essence of stability.

Beyond the random phase approximation: Improved description of short-range correlation by a renormalized adiabatic local density approximation

We assess the performance of a recently proposed renormalized adiabatic local density approximation (rALDA) for \textit{ab initio} calculations of electronic correlation energies in solids and molecules. The method is an extension of the random phase approximation (RPA) derived from time-dependent density functional theory and the adiabatic connection fluctuation-dissipation theorem and contains no fitted parameters. The new kernel is shown to preserve the accurate description of dispersive interactions from RPA while significantly improving the description of short range correlation in molecules, insulators, and metals. For molecular atomization energies the rALDA is a factor of 7(4) better than RPA(PBE) when compared to experiments, and a factor of 3(1.5) better than RPA(PBE) for cohesive energies of solids. For transition metals the inclusion of full shell semi-core states is found to be crucial for both RPA and rALDA calculations and can improve the cohesive energies by up to 0.4 eV. Finally we discuss straightforward generalizations of the method, which might improve results even further.

Photoisomerization and Photoinduced Reactions in Liquid CCl4 and CHCl3

Transient absorption spectroscopy is used to follow the reactive intermediates involved in the first steps in the photochemistry initiated by ultraviolet (266-nm wavelength) excitation of solutions of 1,5-hexadiene, isoprene, and 2,3-dimethylbut-2-ene in carbon tetrachloride or chloroform. Ultraviolet and visible bands centered close to 330 and 500 nm in both solvents are assigned respectively to a charge transfer band of Cl-solvent complexes and the strong absorption band of a higher energy isomeric form of the solvent molecules (iso-CCl3-Cl or iso-CHCl2-Cl). These assignments are supported by calculations of electronic excitation energies. The isomeric forms have significant contributions to their structures from charge-separated resonance forms and offer a reinterpretation of previous assignments of the carriers of the visible bands that were based on pulsed radiolysis experiments. Kinetic analysis demonstrates that the isomeric forms are produced via the Cl-solvent complexes. Addition of the unsaturated hydrocarbons provides a reactive loss channel for the Cl-solvent complexes, and reaction radii and bimolecular rate coefficients are derived from analysis using a Smoluchowski theory model. For reactions of Cl with 1,5-hexadiene, isoprene, and 2,3-dimethylbut-2-ene in CCl4, rate coefficients at 294 K are, respectively, (8.6 ± 0.8) × 10(9), (9.5 ± 1.6) × 10(9), and (1.7 ± 0.1) × 10(10) M(-1) s(-1). The larger reaction radius and rate coefficient for 2,3-dimethylbut-2-ene are interpreted as evidence for an H-atom abstraction channel that competes effectively with the channel involving addition of a Cl-atom to a C═C bond. However, the addition mechanism appears to dominate the reactions of 1,5-hexadiene and isoprene. Two-photon excited CCl4 or CHCl3 can also ionize the diene or alkene solute.

A comparison between state-specific and linear-response formalisms for the calculation of vertical electronic transition energy in solution with the CCSD-PCM method

The calculation of vertical electronic transition energies of molecular systems in solution with accurate quantum mechanical methods requires the use of approximate and yet reliable models to describe the effect of the solvent on the electronic structure of the solute. The polarizable continuum model (PCM) of solvation represents a computationally efficient way to describe this effect, especially when combined with coupled cluster (CC) methods. Two formalisms are available to compute transition energies within the PCM framework: State-Specific (SS) and Linear-Response (LR). The former provides a more complete account of the solute-solvent polarization in the excited states, while the latter is computationally very efficient (i.e., comparable to gas phase) and transition properties are well defined. In this work, I review the theory for the two formalisms within CC theory with a focus on their computational requirements, and present the first implementation of the LR-PCM formalism with the coupled cluster singles and doubles method (CCSD). Transition energies computed with LR- and SS-CCSD-PCM are presented, as well as a comparison between solvation models in the LR approach. The numerical results show that the two formalisms provide different absolute values of transition energy, but similar relative solvatochromic shifts (from nonpolar to polar solvents). The LR formalism may then be used to explore the solvent effect on multiple states and evaluate transition probabilities, while the SS formalism may be used to refine the description of specific states and for the exploration of excited state potential energy surfaces of solvated systems.

Vertical Electronic Excitations in Solution with the EOM-CCSD Method Combined with a Polarizable Explicit/Implicit Solvent Model.

The accurate calculation of electronic transition energies and properties of isolated chromophores is not sufficient to provide a realistic simulation of their excited states in solution. In fact, the solvent influences the solute geometry, electronic structure, and response to external fields. Therefore, a proper description of the solvent effect is fundamental. This can be achieved by combining polarizable explicit and implicit representations of the solvent. The former provides a realistic description of solvent molecules around the solute, while the latter introduces the electrostatic effect of the bulk solution and reduces the need of too large a number of explicit solvent molecules. This strategy is particularly appealing when an accurate method such as equation of motion coupled cluster singles and doubles (EOM-CCSD) is employed for the treatment of the chromophore. In this contribution, we present the coupling of EOM-CCSD with a fluctuating charges (FQ) model and polarizable continuum model (PCM) of solvation for vertical excitations in a state-specific framework. The theory, implementation, and prototypical applications of the method are presented. Numerical tests on small solute-water clusters show very good agreement between full EOM-CCSD and EOM-CCSD-FQ calculations, with and without PCM, with differences ≤ 0.1 eV. Additionally, approximated schemes that further reduce the computational cost of the method are introduced and showed to perform well compared to the full method (errors ≤ 0.1 eV).

Crystal structure analysis and first principle investigation of F doping in LiFePO4

This work presents the synthesis of F-doped LiFePO4/C composite by the specific modification of the recently suggested synthesis procedure based on an aqueous precipitation of precursor material in molten stearic acid, followed by a high temperature treatment. Besides the lattice parameters and the primitive cell volume reductions, compared to the undoped sample synthesized under the same conditions, the Rietveld refinement also shows that fluorine ions preferably occupy specific oxygen sites. Particularly, the best refinement is accomplished when fluorine ions occupy O(2) sites exclusively. By means of up-to-date electronic structure and total energy calculations this experimental finding is theoretically confirmed. Such fluorine doping also produces closing of the gap in the electronic structure and consequently better conductivity properties of the doped compound. In addition, the morphological and electrochemical performances of the synthesized powder are fully characterized.

Stratified graphene/noble metal systems for low-loss plasmonics applications

We propose a composite layered structure for tunable, low-loss plasmon resonances, which con- sists of a noble-metal thin film coated in graphene and supported on a hexagonal boron nitride (hBN) substrate. We calculate electron energy loss spectra (EELS) for these structures, and nu- merically demonstrate that bulk plasmon losses in noble-metal films can be significantly reduced, and surface coupling enhanced, through the addition of a graphene coating and the wide-bandgap hBN substrate. Silver films with a trilayer graphene coating and hBN substrate demonstrated sur- face plasmon-dominant spectral profiles for metallic layers as thick as 34 nm. A continued-fraction expression for the effective dielectric function, based on a specular reflection model which includes boundary interactions, is used to systematically demonstrate plasmon peak tunability for a variety of configurations. Variations include substrate, plasmonic metal, and individual layer thickness for each material. Mesoscale calculation of EELS is performed with individual layer dielectric functions as input to the effective dielectric function calculation, from which the loss spectra are directly determined.

First Principle Study for the Melting Properties of Face-Centred-Cubic Aluminum

Ab initio electronic structure optimization and total energy calculations for fcc aluminum are used to study the equation of state (EOS).Through fitting to quasi-harmonic Debye model, the thermodynamics properties of different temperatures are calculated. The melting curve at different pressures is presented based on Lindemann measures. The results show that the calculated EOS and the revised melting curve both are in a good agreement with the shock compression and the diamond-anvil-cell (DAC) data. The results illustrate that we can use simple static calculation method which takes less time to gain reasonable melting results. It can be used in the qualitative forecast for materials with difficult experiments.

Electron energy distributions through superdense matter by Monte-Carlo simulations

We have studied energy distribution of fast electrons passing through a highly compressed core plasma for fast ignition research in inertial confinement fusion. Recent PIC calculations indicate that the collective effect of electric and magnetic fields on the transport may be less significant than the binary collisions in the case of a high density fusion pellet. In order to understand the net effect of binary collisions in dense plasma, we calculate electron energy distributions at several viewing angles using an electromagnetic cascade Monte-Carlo simulation, EGS5, for estimation of the contribution of multi collisional process. Here, the construction of physical parameters in the code were taken from the calculation results given by 2 dimensional particle-in-cell simulations. In the result, the number of electrons detected on the laser axis within the range to 15 MeV significantly decreases for the superdense region (max: 1.6 · 10 25 (/cm 3 )) compared with the low density plasma. The reduction on the electron number decreases with increase of observation angles gradually and finally the number almost coincides more than 40 degrees.

Performance of recent and high-performance approximate density functionals for time-dependent density functional theory calculations of valence and Rydberg electronic transition energies

We report a test of 30 density functionals, including several recent ones, for their predictions of 69 singlet-to-singlet excitation energies of 11 molecules. The reference values are experimental results collected by Caricato et al. for 30 valence excitations and 39 Rydberg excitations. All calculations employ time-dependent density functional theory in the adiabatic, linear-response approximation. As far as reasonable, all of the assignments are performed by essentially the same protocol as used by Caricato et al., and this allows us to merge our mean unsigned errors (MUEs) with the ones they calculated for both density functional and wave function methods. We find 21 of the 30 density functionals calculated here have smaller MUEs for the 30 valence states than what they obtained (0.47 eV) for the state-of-the-art EOM-CCSD wave function. In contrast, for all of density functionals the MUE for 39 Rydberg states is larger than that (0.11 eV) of EOM-CCSD. Merging the 30 density functionals calculated here with the 26 calculated by Caricato et al. makes a set of 56 density functionals. Averaging the unsigned errors over both the valence excitations and the Rydberg excitations, none of the 56 density functionals shows a lower mean unsigned error than that (0.27 eV) of EOM-CCSD. Nevertheless, two functionals are successful in having an overall mean unsigned error of 0.30 eV, and another nine are moderately successful in having overall mean unsigned errors in the range 0.32-0.36 eV. Successful or moderately successful density functionals include seven hybrid density functionals with 41% to 54% Hartree-Fock exchange, and four range-separated hybrid density functionals in which the percentage of Hartree-Fock exchange increases from 0% to 19% at small interelectronic separation to 65%-100% at long range.

Electron energy response of NaI:Tl and SrI2:Eu calculated from carrier mobilities and measured first- and third-order quenching

Abstract

Photon activation therapy of RG2 glioma carrying Fischer rats using stable thallium and monochromatic synchrotron radiation

75 RG2 glioma-carrying Fischer rats were treated by photon activation therapy (PAT) with monochromatic synchrotron radiation and stable thallium. Three groups were treated with thallium in combination with radiation at different energy; immediately below and above the thallium K-edge, and at 50 keV. Three control groups were given irradiation only, thallium only, or no treatment at all. For animals receiving thallium in combination with radiation to 15 Gy at 50 keV, the median survival time was 30 days, which was 67% longer than for the untreated controls (p = 0.0020) and 36% longer than for the group treated with radiation alone (not significant). Treatment with thallium and radiation at the higher energy levels were not effective at the given absorbed dose and thallium concentration. In the groups treated at 50 keV and above the K-edge, several animals exhibited extensive and sometimes contra-lateral edema, neuronal death and frank tissue necrosis. No such marked changes were seen in the other groups. The results were discussed with reference to Monte Carlo calculated electron energy spectra and dose enhancement factors.

Energetic consideration of the conduction type of Mg2Si doped with Cu, Ag, or Au using first-principle calculations

Electronic energy calculations of Cu-, Ag-, or Au-doped Mg2Si, expected thermoelectric energy conversion materials, have been performed using a density-functional theory assuming that doped elements will work as either (1), substitutional impurities of Mg or Si sites of the Mg2Si lattice, or (2), inserted impurities into its 4b sites. From the energetic point of view, it was found that Cu will be most favorably inserted into the 4b site of Mg2Si, resulting in making a p-type compound. Au will substitute Si of Mg2Si lattice to make an n-type compound. Ag will make both of an interstitial compound and a substitutional compound where Mg and Si are almost equally replaced, but the overall conduction type would be p-type.These predictions agrees well to the observed facts for Ag- and Au- doped Mg2Si but does not for Cu-doped Mg2Si, perhaps because of insufficient solubility of Cu into Mg2Si to compensate the n-type carriers of undoped Mg2Si.

A TD‐DFT basis set and density functional assessment for the calculation of electronic excitation energies of fluorene

AbstractConjugated organic materials are the subject of intensive research for a range of optoelectronic applications. A model for such molecules is fluorene, which consists of rigid planar biphenyl units of C2v symmetry. A low energy experimental absorption spectrum in the gas phase is composed of A1 and B2 transitions. The aim of this work is to evaluate the performance of the basis sets cc‐pVXZ (X = D and T), aug‐cc‐pVDZ, 6‐31G**, 6‐31++G**, 6‐311G**, 6‐311++G**, Sadlej‐pVTZ, Z2Pol, Z3Pol, and pSBKJC and of the functionals B3LYP, B3LYP/CS00, CAM‐B3LYP, PBE0, and LB94 in predicting the electronic transitions obtained taking linear response‐coupled cluster singles and doubles (LR‐CCSD) results as the theoretical reference. Our findings suggest that the time‐dependent density functional theory singles method is not able to correctly assign the predicted spectrum while LR‐CCSD always correctly describes the experimental data. Among the studied density functionals, the best performance was achieved by the CAMB3LYP. For transitions above 5 eV, diffuse functions are required to properly predict the observed transitions. © 2012 Wiley Periodicals, Inc.