Electronic Structure Techniques Research Articles

On the Vertical and Adiabatic Excitation Energies of the 2Ag State of trans-1,3-Butadiene

The excitation energy to the 21Ag state of trans-1,3-butadiene is examined using a variety of ab initio electronic structure techniques. While analogous states have been shown to be the lowest singlet excited states for all longer polyenes, for butadiene the position of the 21Ag state relative to the HOMO → LUMO excitation (11Bu) has been difficult to establish theoretically. We employ a variety of methods (CASSCF, CASPT2, MRSDCI, QDVPT) to examine both the vertical and adiabatic excitation energies for this state. At the ground-state geometry, the vertical excitation energies obtained by CASPT2 and Davidson-corrected MRSDCI for the 21Ag state differ by approximately 0.15 eV, but all of the methods predict that the 21Ag state has a lower 0−0 excitation energy than the 11Bu state. Possible reasons for the discrepancies between the various methods for the vertical excitation energy are discussed.

Theoretical studies of pentene cracking on zeolites: C–C β-scission processes

The C–C bond-breaking step of pentene adsorbed on a model zeolite cluster is examined using ab initio and density functional theory (DFT/B3LYP) electronic structure techniques as an example of the β-scission process that arises in cracking of alkanes and alkenes. After pentene has been protonated by the acid site, the reactant for the cracking process corresponds to a pentyl cation covalently bound to the oxygen of the zeolite, ZO −–C 5H + 11. The product of the C–C bond-breaking process is propene plus an ethyl cation bound to a neighboring oxygen. The energy of the transition state relative from B3LYP calculations is 60 kcal/mol relative to the pentyl cationic reactant. For the case of the branched olefin methyl pentene, the transition state energy is slightly lower (55 kcal/mol), but the overall reaction energy is essentially the same as for pentene. The results are compared to the case of the gas phase pentyl carbenium ion.

First principles modeling of magnetic random access memory devices (invited)

Giant magnetoresistance (GMR) and spin-dependent tunneling may be used to make magnetic random access memory devices. We have applied first-principles based electronic structure techniques to understand these effects and in the case of GMR to model the transport properties of the devices.

Noncollinear magnetism in substitutionally disordered face-centered-cubic FeMn

We use first principles electronic structure techniques to study the magnetic structure of γ-FeMn using the Korringa–Kohn–Rostocker multiple-scattering approach in conjunction with an extension of the single site coherent potential approximation to noncollinear magnetic structures. Our results show that the noncollinear 3Q and 2Q structures are both stable solutions with the former being slightly lower in energy. The collinear solutions could only be converged in a traditional spin-polarized calculation and are unstable in a noncollinear treatment.

Hydrolysis of Ferric Ion in Water and Conformational Equilibrium

Reported here are results of theoretical calculations on Fe(H2O)63+, Fe(H2O)5(OH)2+, three isomers of Fe(H2O)4(OH)2+, and Fe(H2O)3(OH)2+, which investigate the molecular mechanisms of hydrolysis of ferric ion in water. The combination of density functional electronic structure techniques and a dielectric continuum model for electrostatic solvation applied to the Fe(H2O)63+ complex yields an estimate of −1020 kcal/mol (experimental values −1037 to −1019 kcal/mol) for the absolute free energy of the aqueous ferric ion. The predicted free energy change for the first hydrolysis reaction is surprisingly close to the experimental value (2 kcal/mol predicted compared to 3 kcal/mol experimental). For the second hydrolysis reaction, we found an unexpected low-energy isomer of Fe(H2O)4(OH)2+ with five ligands in the inner sphere and one water outside. The hexacoordinate cis and trans isomers are, respectively, slightly lower and higher in energy. Calculations on the pentacoordinate species Fe(H2O)3(OH)2+ suggest that extrusion of the outer-sphere water is nearly thermoneutral. The reaction free energy for the second hydrolysis is predicted in the range 16−18 kcal/mol, higher than the experimental value of 5 kcal/mol. Because the theoretical predictions are higher than experimental values, and novel structures were encountered among products of the second hydrolysis, we argue that conformational entropy is an important omission in this theoretical treatment of net reaction free energies. A fuller cataloging of low-energy hydrolysis products and direct calculations of partition functions of the isolated complexes should help in modeling equilibrium speciation in groundwaters.

Atomistic simulation of thermodynamic and mechanical properties of metals

This paper reviews the atomistic simulation of thermodynamic and mechanical properties of prototype metals using first-principles density-functional methods. Electronic-structure techniques developed from linear muffin-tin orbital theory can yield accurate cold and electron-thermal contributions to the equation of state, pressure-induced structural phase transitions, phonons, elastic moduli, and ideal shear strength to ultrahigh pressures. Complementary interatomic-potentials methods developed from generalized pseudopotential theory can provide in addition the ion-thermal equation of state, high-pressure melting curve, and temperature–pressure phase diagram, as well as the atomic structure and energetics of point and extended defects including vacancies, grain boundaries, and dislocations. The latter represent essential atomistic input into the rigorous multiscale modeling of mechanical properties such as plastic flow. Applications to the metals magnesium, molybdenum, and tantalum are discussed.

Molecular dynamics simulations of compressed liquid hydrogen

Molecular dynamics simulations have been performed for highly compressed fluid hydrogen in the density and temperature regime of recent shock-compression experiments. Both density functional and tight-binding electronic structure techniques have been used to describe interatomic forces. A new tight-binding model of hydrogen has been developed with a single s orbital on each atom that reproduces properties of the dimer, of various crystalline structures, and of the fluid. The simulations give pressures and electrical conductivities in general agreement with the measured values. The pressures are also compared with recent quantum Monte Carlo results. This analysis provides a firm foundation for exploring the origins of the rapid change in electrical conductivity with temperature and density observed in the experiments. The simulations indicate that the conductivity in fluid hydrogen in this regime arises both from: (1) closing of the band gap due to thermal effects and compression; (2) electron hopping facilitated by the dissociated atoms (monomers) with the latter process the most important. Finally, we find that the internal structure of cool, dense hydrogen has a pronounced time-dependent nature with molecules (dimers) constantly dissociating and atoms (monomers) constantly associating all of the time.

Nonadiabatic molecular dynamics simulation of ultrafast pump-probe experiments on I2 in solid rare gases

Recent experimental studies of both A and B state photoexcitation of I2 and the ensuing many-body dynamics in rare gas matrices by Apkarian and co-workers are simulated using the methods we presented in an earlier work combining nonadiabatic molecular dynamics with semiempirical diatomics-in-molecules (DIM) excited state electronic structure techniques. We extend our DIM methods to compute the ion pair states of the I2-rare gas crystal system and use these states together with a model of the configurational dependence of the electronic dipole operator matrix elements to calculate the time resolved probe absorption signals in these pump - probe experiments using a simple golden rule result. Our computed signals are in remarkable agreement with experiments and we use our calculations to provide a detailed microscopic analysis of the channels to predissociation and recombination underlying these experiments.

Thermodynamic properties of FCC-based Al-Ag alloys

The energetics and thermodynamics of fcc-based Al-Ag alloys have important consequences for the formation of Guinier-Preston (GP) zones in Al-rich alloys. Bulk thermodynamic properties of stable and metastable fcc-based phases in the Ag-Al system are studied from first principles using quantum-mechanical, total-energy methods combined with the cluster-variation method. From the results of these calculations the metastable fcc-based Ag-Al phase diagram has been calculated. Our calculated phase diagram features a monotectoid reaction. A miscibility gap in the fcc solid solution phase is present above the monotectoid temperature and a two-phase field between the Al-rich solid solution and an ordered fcc-based Ag 2Al phase is predicted for low temperatures. Therefore, our calculations provide clear evidence that the structure of ε (high temperature) and η (low temperature) GP-zone precipitates should be disordered and long-range ordered, respectively. Our preliminary findings using a full-potential electronic-structure technique indicate that lattice relaxations in the alloys can significantly alter the formation energies.

Vibrational broadening of x-ray emission spectra: A first-principles study on diamond

We present a general theoretical scheme that allows an accurate account of the vibrational broadening of core-level line shapes in crystals, using state-of-the-art electronic-structure techniques within the Franck-Condon approximation. Electronic and vibrational degrees of freedom are consistently treated using ab initio density-functional theory. Neglecting the core-hole lifetime, anharmonic effects are treated within a self-consistent phonon approach. Finite core-hole lifetime effects are simulated by an independent-boson model, which, however, we have been able to apply only when lattice distortions are treated in the harmonic approximation. We apply the theory to the diamond 1s core-level emission spectrum. In the limit of infinite core-hole lifetime, we find that (i) the Stokes shift is about 3 eV, (ii) the harmonic approximation overestimates the Stokes shift by \ensuremath{\sim}1.4 eV, and (iii) phonon broadening is about 2 eV. A comparison with experiment reveals that the phonon bandwidth is comparable to the core-hole lifetime, which we estimate \ensuremath{\Gamma}\ensuremath{\approx}110 meV.

The metallic bond: Elastic properties

Establishing relationships between electronic structure and the properties of metals and alloys has relied heavily on assigning various attributes to the metallic bond. Principal among these is that of a directional bond. Bond directionality has been evoked as an explanation for the magnitudes of elastic shear constants, elastic anisotropy, the susceptibility to brittle failure, and preferred allotropic and alloy structure. Though bond directionality is frequently used as an explanation of metallic and alloy properties, it has never been rigorously defined. In this paper, a definition of bond directionality is provided which allows for its quantification. Using this definition, it is shown that the relative magnitudes of the single crystal shear elastic constants for f.c.c. metals can be correlated with bond directionality. The only information which is required to provide a measure of bond directionality is the total charge density corresponding to the given metal or alloy. This information can now be easily determined through the use of modern electronic structure techniques.

Massively Parallel Self-Consistent-Field Calculations

The advent of supercomputers with many computational nodes each with its own independent memory makes possible very fast computations. Our work is focused on the development of electronic structure techniques for the solution of Grand Challenge-size molecules containing hundreds of atoms. Our efforts have resulted in a scalable direct-SCF (self-consistent-field) Fock matrix construction kernel that is portable and efficient. Good parallel performance is obtained by allowing asynchronous communications and using a distributed data model. These requirements are easily handled by using the Global Array (GA) software library developed for this project. This algorithm has been incorporated into the new program NWChem.

Theoretical studies of the structures and electronic properties of U(NH 2) 3 and Np(NH 2) 3

The electronic structure of the model compounds U(NH 2) 3 and Np(NH 2) 3 is investigated with the aid of ab initio electronic structure techniques. The electronic ground states and equilibrium geometries of these complexes are determined using multiconfiguration SCF techniques. The ground state and low-lying excited states of U(NH 2) 3 are found to arise from the (5f) 2 (6d) 1 and (5f) 3 configurations on the metal. The lowest state of the (5f) 2 (6d) 1 and (5f) 3 configurations are comparable in energy. Np(NH 2) 3 utilizes the (5f) 4 configuration, with the (5f) 3 (6d) 1 configuration lying about 2 eV higher in energy. Comparisons are made to the AnL 3 counterparts with LCH 3.

Studies of molecular Rydberg states by Schwinger variational-quantum defect methods: Application to molecular hydrogen

An ab initio electronic structure technique has been developed to study highly excited states of molecules by combining Schwinger variational methods of collision theory with generalized quantum defect theory. The technique exploits methods of scattering theory to study the region of highly excited Rydberg levels below and across ionization thresholds for molecules. The reaction matrix K, which describes the interaction of the Rydberg electron with the ionic core, is found at arbitrary negative electron energies by employing an unbounded Coulomb Green’s function in the Lippmann–Schwinger equation for the electronic wave function. Quantal conditions are imposed to obtain discrete molecular energy levels, associated Rydberg wave functions, and quantum defect functions, all as a function of the internuclear distance. Results within the static-exchange approximation for the 1,3Σ+u(1σgnσu) and 1,3Πu(1σgnπu) Rydberg states of H2, for n=2–20 and R=1.2–5.0 a0, are presented and discussed.

Role of d and f orbitals in the geometries of low-valent actinide compounds. Ab initio studies of U(CH3)3, Np(CH3)3, and Pu(CH3)3

While organoactinide compounds are traditionally characterized by high oxidation states and coordination numbers, the synthesis, chemistry, and electronic properties of low-valent actinide complexes have been receiving greater attention in recent years. Specific examples of complexes in the AnL family are represented by U[N(SiMe{sub 3}){sub 2}]{sub 3}, U[CH(SiMe{sub 3}){sub 2}]{sub 3}, and Np and Pu analogues, for which cases the ligands adopt a pyramidal arrangement around the actinide. In this communication, the authors report the results of one of the first studies of such low-valent complexes to be carried out using ab initio electronic structure techniques. Related molecules include lanthanide species of the form LnX{sub 3}, which also adopt pyramidal geometries and which have been the subject of semi-empirical theoretical investigations. Transition metal MX{sub 3} species, by contrast, can exhibit either planar or pyramidal forms, depending on the nature of the metal and the ligand. 12 refs., 1 fig., 1 tab.

A theoretical treatment of thea 3Σ1+ →X1Σ++ spin-forbidden dipole-allowed radiative transition in NO+

The radiative lifetime of the spin-forbidden dipole-allowed transition a 3Σ1+ →X1Σ0++is determined using ab initio electronic structure techniques. This transition acquires intensity as a result of the spin–orbit induced perturbations, a 3Σ+1∼1Π1 and X1Σ0++∼3Π0+ where the notation is meant to imply that the a 3Σ+1 and X1Σ0++ states are perturbed by all the states of the indicated symmetry in a given configuration state function (CSF) space. A near degeneracy of the A 1Π and a 3Σ+ potential energy curves necessitates the use of quasidegenerate perturbation theory when treating the a 3Σ+1∼1Π1 interaction. The nonrelativistic (zeroth order CI), and relativistic (first order perturbed), wave functions are expanded in CSF spaces of dimension 0.5–1.4×106. To our knowledge, this represents the first treatment of the spin–orbit interaction within the full microscopic Breit–Pauli approximation in a CSF space of greater than 1 million CSFs. The radiative lifetimes (τv) of the transitions (a 3Σ+,v)→X 1Σ+ were found to be τv=455, 467, 478, 488, 495 ms for v=0, 1, 2, 3, 4, respectively. These results are in good agreement with the experimental determination of Kuo et al. [J. Chem. Phys. 92, 4849 (1990)] who found τ=530(+300,−100) ms. The results of this treatment are compared with those of a single perturber model in which the a 3Σ+→X 1Σ+ transition is attributed exclusively to the a 3Σ+∼A 1Π perturbation. This model is shown to be only qualitatively correct.

On the electronic structure and dynamical aspects of the predissociation of the A 2ΠΩ states of MgCl. A rigorous quantum mechanical treatment incorporating spin–orbit and derivative coupling effects

The nonadiabatic electronic structure and decay mechanisms of the spectroscopically analyzed A 2Π state of MgCl are studied using a unique combination of rigorous ab initio electronic structure and quantum dynamics techniques. The electronic structure treatment is based on ab initio multiconfiguration self-consistent field/first order and second-order configuration interaction wave functions [ΨaJ(r;R)] for the 1,2 2Π adiabatic electronic states. These wave functions are used to determine rigorous diabatic state wave functions using the derivative coupling matrix elements, gaJ,I(R)=〈ΨaJ(r;R) ‖(∂/∂R)ΨaI(r;R)〉r, and approximate diabatic state wave functions using a diabatization procedure based on diagonalizing the dipole moment operator suggested by Werner and Meyer [J. Chem. Phys. 74, 5802 (1981)]. Near its equilibrium geometry the A 2ΠΩ state is well characterized as a regular diabatic state corresponding to Mg+(2P)-Cl−(1S). The higher vibrational levels of this state are significantly perturbed (predissociated) by the interaction with a second inverted, dissociative, diabatic state of the same symmetry corresponding to Mg0(1S)Cl0(2P). Because the fine structure splitting, E(2 2Π3/2)–E(2 2Π1/2), of the predissociating state is comparable to the vibrational spacing of the predissociated state, relativistic, spin–orbit, effects must be incorporated into the treatment of the predissociation. These relativistic effects are treated using the full microscopic Breit–Pauli spin–orbit (that is spin–orbit and spin–other–orbit contributions) operator. The coupled diabatic state representation incorporating relativistic effects is used as the basis for semiclassical and fully quantum mechanical treatments of the predissociation lifetimes and energy shifts of the A 2Π3/2,1/2 vibrational states. By correctly taking into account the mixing of the regular and inverted 1,2 2Π states the coupled state quantum scattering calculations are able to reproduce the observed ‘‘anomalous’’ dependence of the fine structure splitting factor Asov.

Empirical Interatomic Potentials for L1O Tial and B2 Nial

ABSTRACTRecent studies have suggested a particular relationship between the degree of covalent bonding in TiAl and the mobility of dislocation[1,2]. Ultimately such electronic effects In ordered compounds must dictate the dislocation core structures and at the same time the dislocation mobility within a given compound. However, direct modelling of line defects Is beyond the capability of todays electronic structure techniques. Alternatively, significant steps toward extending our understanding of the flow behaviour of structural intermetallics may come through general application of empirical interatomic potential methods for calculating the structure and mobility of defects. Toward this end, we have constructed semi-empirical interatomic potentials within the embedded atom formalism for L1O and B2 type structures. These potentials have been determined by fitting to known bulk structural and elastic properties of TIAl and NiAl, using least squares procedures. Simple expressions that relate the parameters of the potentials to the bulk properties are used in the fitting procedure. Calculations of dislocation core structures and planar fault energies using these potentials are considered. The differences between the optimized bulk properties predicted from the potentials and the values for these properties are discussed in terms of non-spherical nature of the electron density distribution. Empirical methods which incorporate these effects into interatomic potentials are briefly discussed.

Role of metal d states in II-VI semiconductors.

All-electron band-structure calculations and photoemission experiments on II-VI semiconductors both exhibit a metal d subband inside the main valence band. It has nevertheless been customary in pseudopotential and tight-binding approaches to neglect the metal d band by choosing Hamiltonian parameters which place this band inside the chemically inert atomic cores. Using all-electron self-consistent electronic-structure techniques (which treat the outermost d electrons on the same footing as other valence electrons) and comparing the results to those obtained by methods which remove the d band from the valence spectrum, we study their effects on valence properties. For II-VI semiconductors we find that p-d repulsion and hybridization (i) lower the band gaps, (ii) reduce the cohesive energy, (iii) increase the equilibrium lattice parameters, (iv) reduce the spin-orbit splitting, (v) alter the sign of the crystal-field splitting, (vi) increase the valence-band offset between common-anion II-VI semiconductors, and (vii) modify the charge distributions of various II-VI systems and their alloys. p-d repulsion is also shown to be responsible for the occurrence of deep Cu acceptor levels in II-VI semiconductors (compared with shallow acceptors of Zn in III-V), for the anomalously small band gaps in chalcopyrites, and for the negative exchange splitting in ferromagnetic MnTe.

Ordered-vacancy-compound semiconductors: Pseudocubic CdIn2Se4.

Whereas substitutional adamantine compounds ${A}_{n}$${B}_{4\mathrm{\ensuremath{-}}n}$${C}_{4}$ (e.g., ${\mathrm{ABC}}_{2}$ chalcopyrites for n=2, or the AC and BC zinc-blende compounds for n=4 and 0) have four metal atoms around each nonmetal atom and vice versa, ordered-vacancy compounds OVC's) ${\mathrm{AB}}_{2}$${C}_{4}$ have but three metal atoms (one A and two B's) around each nonmetal site (C) while the fourth (unoccupied) site forms an ordered array of vacancies. An example for OVC's is ``pseudocubic'' ${\mathrm{CdIn}}_{2}$${\mathrm{Se}}_{4}$ which can be structurally derived from the layered alternate monolayer superlattice of CdSe and InSe (along the [001] direction) by removing half of the Cd atoms from each Cd plane. Such OVC's form a natural bridge between crystal and impurity physics. Much like the metal vacancy in II-VI compounds (e.g., CdSe), the vacancy in ${\mathrm{CdIn}}_{2}$${\mathrm{Se}}_{4}$ has associated with it (nonmetal) ``dangling bonds'' and ``lone-pair'' electrons, which, however, form a dispersed band in the crystal. Using all-electron mixed-basis electronic-structure techniques, we study the properties of such an ordered array of vacancies in ${\mathrm{CdIn}}_{2}$${\mathrm{Se}}_{4}$ vis-a$ga----vis the experimental data. We find vacancy-induced atomic relaxations (Se moves towards the vacant site), resonant broadening of the lone-pair dangling-bond states into a \ensuremath{\approxeq}3-eV band, and that the total charge density around the vacant site has little density and shows scant evidence of dangling bonds. We discuss the nature of the bonding in this system, comparing it to other covalent selenides and to the observed photoemission and optical data. A number of possible order-disorder transitions, including the disordering of cations on the vacant sites, are identified.