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Document Server@UHasselt (58.026 recursos)

Repository of the University of Hasselt containing publications in the fields of statistics, computer science, information strategies and material from the Institute for behavioural sciences.

Publications

Mostrando recursos 1 - 20 de 265

  1. Quenching of magnetism in hexagonal graphene nanoflakes by non-local electron correlation

    HAJGATO, Balazs; DELEUZE, Michael
    Non-local MP2-like electron correlation in DFT calculations employing the double hybrid dispersion corrected B2PLYD functional is found to systematically quench anti-ferromagnetism in a variety of hexagonal graphene nanoflakes. The scaling in size of the associated spin contamination demonstrates further the artefactual character of any spin-polarization of electron densities in singlet electronic states.

  2. Benchmark theoretical study of the ionization energies, electron affinities and singlet-triplet energy gaps of azulene, phenanthrene, pyrene, chrysene and perylene

    HUZAK, Matija; HAJGATO, Balazs; DELEUZE, Michael
    The vertical and adiabatic singlet-triplet energy gaps, electron affinities and ionization energies of azulene, phenanthrene, pyrene, chrysene, and perylene are computed by applying the principles of a focal point analysis onto a series of single-point calculations at the level of Hartree-Fock theory, second-, third-, and fourth-order Moller-Plesset perturbation theory, as well as coupled cluster theory including single, double and perturbative triple excitations, in conjunction with correlation consistent basis sets of improving quality. Results are supplemented with an extrapolation to the limit of an asymptotically complete basis set. According to our best estimates, azulene, phenanthrene, pyrene, chrysene, and perylene exhibit adiabatic...

  3. Electron momentum spectroscopy of metal carbonyls: a reinvestigation of the role of nuclear dynamics

    Hajgato, Balazs; Morini, Filippo; Deleuze, Michael
    The main purpose of this work is to reinvestigate the influence of nuclear dynamics in the electronic ground state of group 6 metal hexacarbonyl compounds [W(CO)6, Cr(CO)6, Mo(CO)6] on electron momentum density profiles obtained from experimental orbital reconstructions employing Electron Momentum Spectroscopy. We call into questionthe view (Liu et al. in Chem Phys Lett 497:229, 2010) that thermally induced nuclear displacements associated with the first three triply degenerate 1T2g, 1T1u, and 1T2u vibrational eigenmodes can be large enough at or near room temperature (298–310K) to explain on their own the unexpectedly large electron densities inferred for the frontier orbitals of...

  4. The band 12 issue of norbornane: A study of higher shake-up states

    Knippenberg, Stefan; HAJGATO, Balazs
    In line with a recent study of the electronic structure of the cage compound norbornane (J. Chem. Phys. 121 (2004), 10525:J. Phys. Chem. A 109 (2005), 4267), symmetry adapted cluster expansion configuration interaction (SAC-CI) general R calculations have been performed and compared with results obtained by the third order algebraic diagrammatic construction scheme [ADC(3)]. Comparison has been made with previously performed electron momentum spectroscopy (EMS) and ultraviolet photo-electron measurements. The region around similar to 25eV (band 12), characterized by an elaborated band in the EMS spectrum which is missing in previous Green's function and ADC calculations, is investigated. This study...

  5. Focal Point Analysis of the Singlet-Triplet Energy Gap of Octacene and Larger Acenes

    Hajgato, Balazs; Huzak, Matija; Deleuze, Michael S.
    A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet-triplet (ST) excitation energies of n-acenes (C4n+2H2n+4) ranging from octacene (n = 8) to undecacene (n = 11) is presented. The T1 diagnostics of coupled cluster theory and further energy-based criteria demonstrate that all investigated systems exhibit predominantly a 1Ag singlet closed-shell electronic ground state. Singlet-triplet (S0-T1) energy gaps can therefore be very accurately determined by applying the principle of a focal point analysis (FPA) onto the results of a series of single-point and symmetry-restricted calculations employing correlation consistent cc-pVXZ basis sets (X = D,...

  6. Half-metallicity and spin-contamination of the electronic ground state of graphene nanoribbons and related systems: An impossible compromise?

    Huzak, Matija; Deleuze, Michael S.; Hajgato, Balazs
    An analysis using the formalism of crystalline orbitals for extended systems with periodicity in one dimension demonstrates that any antiferromagnetic and half-metallic spin-polarization of the edge states in n-acenes, and more generally in zigzag graphene anoislands and nanoribbons of finite width, would imply a spin contamination that increases proportionally to system size, in sharp and clear contradiction with the implications of Lieb’s theorem for compensated bipartite lattices and the expected value for a singlet (S = 0) electronic ground state. Verifications on naphthalene, larger n-acenes (n = 3–10) and rectangular nanographene islands of increasing size, as well as a...

  7. Photoelectron and electron momentum spectroscopy of 1-butene at benchmark theoretical levels

    Shojaei, S.H.Reza; Morini, Filippo; Hajgato, Balazs; Deleuze, M. S.
    The results of experimental studies of the valence electronic structure of 1-butene employing photoelectron spectroscopy as well as electron momentum spectroscopy are interpreted on the ground of quantitative calculations of one-electron and shake-up ionization energies and of the related Dyson orbitals, using one-particle Green's function theory in conjunction with the third-order algebraic diagrammatic construction scheme (ADC(3)). Comparison is made with simulations of (e, 2e) electron momentum distributions obtained from standard (B3LYP) Kohn-Sham orbitals. Our analysis is based on highly quantitative determinations of the energy difference between the cis and gauche (C(1)) conformers, within similar to 0.02 kcal mol(-1) accuracy, and...

  8. Photoelectron and Electron Momentum spectroscopy of 1-Butene at Benchmark Theoretical Levels

    Shojaei, S.H.Reza; Morini, Filippo; Hajgato, Balazs; Deleuze, Michael S.
    The results of experimental studies of the valence electronic structure of 1-butene employing photoelectron spectroscopy as well as electron momentum spectroscopy are interpreted on the ground of quantitative calculations of one-electron and shake-up ionization energies and of the related Dyson orbitals, using one-particle Green’s function theory in conjunction with the third-order algebraic diagrammatic construction scheme (ADC(3)). Comparison is made with simulations of (e, 2e) electron momentum distributions obtained from standard (B3LYP) Kohn–Sham orbitals. Our analysis is based on highly quantitative determinations of the energy difference between the cis and gauche (C1) conformers, within ∼0.02 kcal mol−1 accuracy, and a thermostatistical evaluation thereby of conformer weights beyond the level...

  9. Electron Momentum Spectroscopy of pyrimidine at the benchmark ADC(3) level

    Shojaei, S.H.Reza; Hajgato, Balazs; Deleuze, Michael S.
    An extensive study of the valence electronic structure, ionization spectrum and electron momentum distributions of pyrimidine is presented, on the ground of accurate calculations of one-electron and shakeup ionization energies and of the related Dyson orbitals, using one-particle Green’s Function theory in conjunction with the third-order Algebraic Diagrammatic Construction scheme [ADC(3)]. Comparison is made with results obtained from standard (B3LYP) Kohn–Sham orbitals and recent measurements employing Electron Momentum Spectroscopy. Quantitative insights into the experimental momentum distributions are amenable without resorting to any empirical rescaling of pole strengths, provided band overlaps and relaxation effects are properly accounted for.

  10. Photoelectron and Electron Momentum Spectroscopy of 1-Butene at Benchmark Theoretical Levels

    Shojaei, S.H.Reza; Morini, Filippo; Hajgató, Bálazs; Deleuze, Michael S.
    The results of experimental studies of the valence electronic structure of 1-butene employing photoelectron spectroscopy as well as electron momentum spectroscopy are interpreted on the ground of quantitative calculations of one-electron and shake-up ionization energies and of the related Dyson orbitals, using one-particle Green's function theory in conjunction with the third-order algebraic diagrammatic construction scheme (ADC(3)). Comparison is made with simulations of (e, 2e) electron momentum distributions obtained from standard (B3LYP) Kohn–Sham orbitals. Our analysis is based on highly quantitative determinations of the energy difference between the cis and gauche (C1) conformers, within ~0.02 kcal mol−1 accuracy, and a thermostatistical...

  11. Many-body calculations of molecular electric polarizabilities in asymptotically complete basis sets

    Monten, Ruben; HAJGATO, Balazs; DELEUZE, Michael
    The static dipole polarizabilities of Ne, CO, N(2), F(2), HF, H(2)O, HCN, and C(2)H(2) (acetylene) have been determined close to the Full-CI limit along with an asymptotically complete basis set (CBS), according to the principles of a Focal Point Analysis. For this purpose the results of Finite Field calculations up to the level of Coupled Cluster theory including Single, Double, Triple, Quadruple and perturbative Pentuple excitations [CCSDTQ(P)] were used, in conjunction with suited extrapolations of energies obtained using augmented and doubly-augmented Dunning's correlation consistent polarized valence basis sets of improving quality. The polarizability characteristics of C(2)H(4) (ethylene) and C(2)H(6) (ethane)...

  12. Half-metallicity and spin-contamination of the electronic ground state of graphene nanoribbons and related systems: An impossible compromise?

    HUZAK, Matija; DELEUZE, Michael; HAJGATO, Balazs
    An analysis using the formalism of crystalline orbitals for extended systems with periodicity in one dimension demonstrates that any antiferromagnetic and half-metallic spin-polarization of the edge states in n-acenes, and more generally in zigzag graphene nanoislands and nanoribbons of finite width, would imply a spin contamination < S(2)> that increases proportionally to system size, in sharp and clear contradiction with the implications of Lieb's theorem for compensated bipartite lattices and the expected value for a singlet (S = 0) electronic ground state. Verifications on naphthalene, larger n-acenes (n = 3-10) and rectangular nanographene islands of increasing size, as well as...

  13. Focal Point Analysis of the Singlet-Triplet Energy Gap of Octacene and Larger Acenes

    HAJGATO, Balazs; HUZAK, Matija; DELEUZE, Michael
    A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet triplet (ST) excitation energies of n-acenes (C(4n+2)H(2n+4)) ranging from octacene (n = 8) to undecacene (n = 11) is presented. The T1 diagnostics of coupled cluster theory and further energy-based criteria demonstrate that all investigated systems exhibit predominantly a (1)A(g) singlet closed-shell electronic ground state. Singlet-triplet (S(0)-T(1)) energy gaps can therefore be very accurately determined by applying the principle of a focal point analysis (FPA) onto the results of a series of single-point and symmetry-restricted calculations employing correlation consistent cc-pVXZ basis sets (X =...

  14. Nonequilibrium wetting transition in a nonthermal 2D Ising model

    HOOYBERGHS, Jef; Indekeu, J. O.
    Nonequilibrium wetting transitions are observed in Monte Carlo simulations of a kinetic spin system in the absence of a detailed balance condition with respect to an energy functional. A nonthermal model is proposed starting from a two-dimensional Ising spin lattice at zero temperature with two boundaries subject to opposing surface fields. Local spin excitations are only allowed by absorbing an energy quantum (photon) below a cutoff energy E (c) . Local spin relaxation takes place by emitting a photon which leaves the lattice. Using Monte Carlo simulation nonequilibrium critical wetting transitions are observed as well as nonequilibrium first-order wetting phenomena,...

  15. Bonding in negative ions: the role of d orbitals in the heavy analogues of pyridine and furan radical anions

    HAJGATO, Balazs; De Proft, Frank; Szieberth, Denes; Tozer, David J.; DELEUZE, Michael; Geerlingsa, Paul; Nyulaszi, Laszlo
    We have used a potential wall method to investigate the role of d orbitals in the a(2) singly-occupied molecular orbitals of (2)A(2) negative ion states of two molecular series: pyridine, phosphabenzene, arsabenzene, stibabenzene (C5H5X, X = {N, P, As, Sb}), and furan, thiophene, selenophene, tellurophene (C4H4X, X = {O, S, Se, Te}). Unlike for the lower lying doubly occupied orbitals, heteroatom d-carbon p in-phase (bonding) interactions in these a(2) orbitals are clearly identified and explain the 0.5 eV stabilization of the (2)A(2) radical anion state in those compounds where the heteroatoms have d orbitals in the valence shell, compared to...

  16. Probing electron correlation and nuclear dynamics in Momentum Space

    DELEUZE, Michael; HAJGATO, Balazs; MORINI, Filippo; KNIPPENBERG, Stefan
    Orbital imaging experiments employing Electron Momentum Spectroscopy are subject to many complications, such as distorted wave effects, conformational mobility in the electronic ground state, ultra-fast nuclear dynamics in the final state, or a dispersion of the ionization intensity over electronically excited (shake-up) configurations of the cation. The purpose of the present contribution is to illustrate how a proper treatment of these complications enables us to probe in momentum space the consequences of electron correlation and nuclear dynamics in neutral and cationic states.

  17. Structure, Stability, and Generation of CH3CNS

    Krebsz, Melinda; HAJGATO, Balazs; Bazso, Gabor; Tarczay, Gyoergy; Pasinszki, Tibor
    The unstable acetonitrile N-sulfide molecule CH3CNS has been photolytically generated in inert solid argon matrix from 3,4-dimethyl-1,2,5-thiadiazole by 254-nm UV irradiation, and studied by ultraviolet spectroscopy and mid-infrared spectroscopy. The molecule is stable in the matrix to 254-nm UV irradiation, but decomposes to CH3CN and a sulfur atom when broad-band UV irradiation is used. Chemiluminescence due to S-2 formation from triplet sulfur atoms was detected on warming the matrix to similar to 20-25 K. The ground-state structure and potential uni- and bimolecular reactions of CH3CNS are investigated using B3LYP, CCSD(T), and MR-AQCC quantum-chemical methods. CH3CNS is demonstrated to be stable...

  18. Electron Momentum Spectroscopy of pyrimidine at the benchmark ADC(3) level

    SHOJAEIKOHNEHSHAHRI, Reza; HAJGATO, Balazs; DELEUZE, Michael
    An extensive study of the valence electronic structure, ionization spectrum and electron momentum distributions of pyrimidine is presented, on the ground of accurate calculations of one-electron and shake-up ionization energies and of the related Dyson orbitals, using one-particle Green's Function theory in conjunction with the third-order Algebraic Diagrammatic Construction scheme [ADC( 3)]. Comparison is made with results obtained from standard (B3LYP) Kohn-Sham orbitals and recent measurements employing Electron Momentum Spectroscopy. Quantitative insights into the experimental momentum distributions are amenable without resorting to any empirical rescaling of pole strengths, provided band overlaps and relaxation effects are properly accounted for. (C) 2010...

  19. Electron Momentum Spectroscopy of Norbornadiene at the Benchmark ADC(3) Level

    MORINI, Filippo; HAJGATO, Balazs; DELEUZE, Michael
    An extensive study, throughout the valence region, of the electronic structure, ionization spectrum, and electron momentum distributions of norbornadiene is presented, on the ground of accurate calculations of valence one-electron and shake-up ionization energies and of the related Dyson orbitals, using one-particle Green's function (1p-GF) theory in conjunction with the so-called third-order algebraic diagrammatic construction scheme [ADC(3)]. Comparison is made with results obtained from standard (B3LYP) Kohn-Sham orbitals and measurements employing electron momentum spectroscopy, taking into account the contamination of inner- and outer-valence spectral bands by numerous shake-up states. Four relatively intense shake-up lines at 12.1, 16.4, 17.6, and 17.8...

  20. Correlation effects in the valence ionization spectra of large conjugated molecules: p-Benzoquinone, anthracenequinone and pentacenequinone

    DELEUZE, Michael; KNIPPENBERG, Stefan
    A review of an extensive series of theoretical studies of the valence one-electron and shake-up ionization spectra of polycyclic aromatic hydrocarbons is presented, along with new results for three planar quinone derivatives, obtained using one-particle Green's function (1p-GF) theory along with the so-called third-order algebraic diagrammatic construction [ADC(3)] scheme and the outer-valence Green's function (OVGF) approximation. These results confirm both for the pi- and sigma-band systems the rapid spreading, upon increasing system size, of many shake-up lines with significant intensities at outer-valence energies. Linear regressions demonstrate that with large conjugated molecules the location of the shake-up onset in the pi-band...

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