Vibrational excitation is now well known to promote the dissociation of H, on Cu surfaces.' ... more Vibrational excitation is now well known to promote the dissociation of H, on Cu surfaces.' This is due to the occurrence of a strong curvature in the interaction potential as the bonding switches from intramolecular to H-surface bonding before the disso-ciation barrier, and a sharp ...
ABSTRACT The strength of the CN/CN′ bond decreases in the series NC-CN (1), CN-CN (2), and CN-NC ... more ABSTRACT The strength of the CN/CN′ bond decreases in the series NC-CN (1), CN-CN (2), and CN-NC (3), while simultaneously it shortens. The explanation requires that, apart from the σ pair bond between the CN 5σ MO's, the following effects be taken into account: (a) Pauli repulsion ("steric hindrance") between the 4σ ("N lone pair") orbitals; (b) Pauli repulsion between the 4σ and 5σ orbitals; (c) donor/acceptor interaction between the 4σ and 5σ orbitals; (d) donor/acceptor interaction between the occupied 1π and unoccupied 2π*. Each of these contributions is numerically significant. The singly occupied 5σ plays, apart from the pair bonding, a dual role, causing Pauli repulsion (with the N lone pair) as an occupied orbital and causing charge-transfer interaction in its capacity of unoccupied acceptor orbital. Detailed consideration of the balance between the repulsive and attractive energy components and their R dependence is required to explain why the central CN/CN′ bond considerably weakens when going from 1 to 3, and why it still contracts. Density functionals that include gradient corrections for the exchange and electron gas parametrization for correlation between electrons of different spin (but no same-spin correlation) are shown to achieve an accuracy for the systems under consideration which is comparable to that of conventional high-level ab initio methods like CEPA and the CCD(+ST) coupled cluster approach.
We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion o... more We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion of H2O into H2O2 in water solution by the FeIVO2+ group under room-temperature and -pressure conditions. We compute the free energy of formation of an O(water)–O(oxo) bond using thermodynamic integration with explicit solvent and we examine the subsequent generation of H2O2 by proton transfer. We show that the O–O bond formation follows the standard reactivity pattern observed in hydroxylation reactions catalyzed by high-spin (S = 2) iron(IV)–oxo species, which is initiated by the transfer of one electron from the highest occupied molecular orbital of the moiety attacking the FeIVO2+ group, either a −C–H bonding orbital (hydroxylation) or a lone pair of a water molecule (water oxidation). The highly electrophilic character exhibited by the FeIVO2+ ion, which is related to the presence of an acceptor 3σ* orbital at low energy with a large contribution on the O end of the FeIVO2+ ion, is the crucial factor promot...
Density Functional Theory of Molecules, Clusters, and Solids, 1996
Traditionally quantum chemical methods use the Hartree-Fock model as a starting point and explici... more Traditionally quantum chemical methods use the Hartree-Fock model as a starting point and explicitly or implicitly employ expansions of the wavefunction in determinants that are generated by orbital substitutions in the HF determinant. Increasingly sophisticated methods for the selection of determinants have been developed, such as threshold-based methods (MRDCI), direct-CI methods and coupled-cluster approaches. The development is driven by both advances in mathematical technique (algebraic as well as diagrammatical) and by computational convenience and feasibility. In contrast, density functional theory (DFT) concentrates on the form of the exchange-correlation part of the total energy expression and on the Kohn-Sham one-electron potential that enters the effective one-electron equations. The present interest in DFT from the (quantum)chemistry community stems from the notable improvement that is obtained over Hartree-Fock, in particular if also non-local corrections are included (cf. other papers in this volume). Evidently, it is desirable to understand precisely which effects of correlation are build into the exchange correlation potential and energy of present day DF appraoches. For two-electron systems such an analysis has been carried out for the exact Kohn-Sham potential by the present authors1. It has been shown that in this case V xc consists of three contributing terms, V cond , V kin and V N−1, that each have a clear physical meaning1. V cond , for instance, is the potential due to the conditional electron density that will be defined below (eq. 1.6): the density of the remaining electrons when one electron (the reference electron) is known to be at a given position r1· The conditional density thus incorporates exactly all effects of correlation (Fermi and Coulomb holes).
We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001)... more We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001) in the incident energy range 78-150 meV, and a theoretical study of dissociative chemisorption of H(2) in the same system. Pronounced out-of-plane diffraction was observed in the whole energy range studied. The energy dependence of the elastic diffraction intensities was measured along the two main symmetry directions for a fixed parallel translational energy. The data were compared with quantum dynamics calculations performed by using DFT-based, six-dimensional potential energy surfaces calculated with both the PW91 and RPBE functionals, as well as with a functional obtained from a weighted average of both (the MIX functional, which was earlier shown to perform quite well for H(2) + Cu(111)). Our results show that the PW91 functional describes the H(2) diffraction intensities more accurately than the RPBE and the MIX functionals, although the absolute values of these intensities are overestimated in the calculations. For the reaction probabilities a preference for one or the other functional cannot be given over the entire energy range probed by the sticking experiments. The PW91 functional yields too high reaction probabilities over the entire investigated energy range, but is better than RPBE at low collision energies (<0.1 eV). The RPBE functional gives too low reaction probabilities at low energy and somewhat too high reaction probabilities at high energy, but agrees better with experiment than PW91 for energies >0.1 eV. The results suggest that, in order to get a better description of both H(2) diffraction and dissociative chemisorption for this system, a specific reaction parameter functional for H(2) + Ru(0001) is needed that is a weighted average of functionals other than PW91 and RPBE. We speculate that differences between the H(2) + Ru(0001) system (early and low reaction barrier) and H(2) + Cu(111) (late and high reaction barrier) may well lead to fundamentally different specific reaction parameter functionals, and that including a reasonable accurate description of the van der Waals interaction might be important for H(2) + Ru(0001) which has barriers localised far away from the surface. Based on our results we advocate new, systematic combined theoretical and experimental studies of H(2) interacting with transition metals in early and late barrier systems, with the aim of determining whether specific reaction parameter functionals for these systems might differ in a systematic way.
Vibrational excitation is now well known to promote the dissociation of H, on Cu surfaces.' ... more Vibrational excitation is now well known to promote the dissociation of H, on Cu surfaces.' This is due to the occurrence of a strong curvature in the interaction potential as the bonding switches from intramolecular to H-surface bonding before the disso-ciation barrier, and a sharp ...
ABSTRACT The strength of the CN/CN′ bond decreases in the series NC-CN (1), CN-CN (2), and CN-NC ... more ABSTRACT The strength of the CN/CN′ bond decreases in the series NC-CN (1), CN-CN (2), and CN-NC (3), while simultaneously it shortens. The explanation requires that, apart from the σ pair bond between the CN 5σ MO's, the following effects be taken into account: (a) Pauli repulsion ("steric hindrance") between the 4σ ("N lone pair") orbitals; (b) Pauli repulsion between the 4σ and 5σ orbitals; (c) donor/acceptor interaction between the 4σ and 5σ orbitals; (d) donor/acceptor interaction between the occupied 1π and unoccupied 2π*. Each of these contributions is numerically significant. The singly occupied 5σ plays, apart from the pair bonding, a dual role, causing Pauli repulsion (with the N lone pair) as an occupied orbital and causing charge-transfer interaction in its capacity of unoccupied acceptor orbital. Detailed consideration of the balance between the repulsive and attractive energy components and their R dependence is required to explain why the central CN/CN′ bond considerably weakens when going from 1 to 3, and why it still contracts. Density functionals that include gradient corrections for the exchange and electron gas parametrization for correlation between electrons of different spin (but no same-spin correlation) are shown to achieve an accuracy for the systems under consideration which is comparable to that of conventional high-level ab initio methods like CEPA and the CCD(+ST) coupled cluster approach.
We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion o... more We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion of H2O into H2O2 in water solution by the FeIVO2+ group under room-temperature and -pressure conditions. We compute the free energy of formation of an O(water)–O(oxo) bond using thermodynamic integration with explicit solvent and we examine the subsequent generation of H2O2 by proton transfer. We show that the O–O bond formation follows the standard reactivity pattern observed in hydroxylation reactions catalyzed by high-spin (S = 2) iron(IV)–oxo species, which is initiated by the transfer of one electron from the highest occupied molecular orbital of the moiety attacking the FeIVO2+ group, either a −C–H bonding orbital (hydroxylation) or a lone pair of a water molecule (water oxidation). The highly electrophilic character exhibited by the FeIVO2+ ion, which is related to the presence of an acceptor 3σ* orbital at low energy with a large contribution on the O end of the FeIVO2+ ion, is the crucial factor promot...
Density Functional Theory of Molecules, Clusters, and Solids, 1996
Traditionally quantum chemical methods use the Hartree-Fock model as a starting point and explici... more Traditionally quantum chemical methods use the Hartree-Fock model as a starting point and explicitly or implicitly employ expansions of the wavefunction in determinants that are generated by orbital substitutions in the HF determinant. Increasingly sophisticated methods for the selection of determinants have been developed, such as threshold-based methods (MRDCI), direct-CI methods and coupled-cluster approaches. The development is driven by both advances in mathematical technique (algebraic as well as diagrammatical) and by computational convenience and feasibility. In contrast, density functional theory (DFT) concentrates on the form of the exchange-correlation part of the total energy expression and on the Kohn-Sham one-electron potential that enters the effective one-electron equations. The present interest in DFT from the (quantum)chemistry community stems from the notable improvement that is obtained over Hartree-Fock, in particular if also non-local corrections are included (cf. other papers in this volume). Evidently, it is desirable to understand precisely which effects of correlation are build into the exchange correlation potential and energy of present day DF appraoches. For two-electron systems such an analysis has been carried out for the exact Kohn-Sham potential by the present authors1. It has been shown that in this case V xc consists of three contributing terms, V cond , V kin and V N−1, that each have a clear physical meaning1. V cond , for instance, is the potential due to the conditional electron density that will be defined below (eq. 1.6): the density of the remaining electrons when one electron (the reference electron) is known to be at a given position r1· The conditional density thus incorporates exactly all effects of correlation (Fermi and Coulomb holes).
We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001)... more We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001) in the incident energy range 78-150 meV, and a theoretical study of dissociative chemisorption of H(2) in the same system. Pronounced out-of-plane diffraction was observed in the whole energy range studied. The energy dependence of the elastic diffraction intensities was measured along the two main symmetry directions for a fixed parallel translational energy. The data were compared with quantum dynamics calculations performed by using DFT-based, six-dimensional potential energy surfaces calculated with both the PW91 and RPBE functionals, as well as with a functional obtained from a weighted average of both (the MIX functional, which was earlier shown to perform quite well for H(2) + Cu(111)). Our results show that the PW91 functional describes the H(2) diffraction intensities more accurately than the RPBE and the MIX functionals, although the absolute values of these intensities are overestimated in the calculations. For the reaction probabilities a preference for one or the other functional cannot be given over the entire energy range probed by the sticking experiments. The PW91 functional yields too high reaction probabilities over the entire investigated energy range, but is better than RPBE at low collision energies (<0.1 eV). The RPBE functional gives too low reaction probabilities at low energy and somewhat too high reaction probabilities at high energy, but agrees better with experiment than PW91 for energies >0.1 eV. The results suggest that, in order to get a better description of both H(2) diffraction and dissociative chemisorption for this system, a specific reaction parameter functional for H(2) + Ru(0001) is needed that is a weighted average of functionals other than PW91 and RPBE. We speculate that differences between the H(2) + Ru(0001) system (early and low reaction barrier) and H(2) + Cu(111) (late and high reaction barrier) may well lead to fundamentally different specific reaction parameter functionals, and that including a reasonable accurate description of the van der Waals interaction might be important for H(2) + Ru(0001) which has barriers localised far away from the surface. Based on our results we advocate new, systematic combined theoretical and experimental studies of H(2) interacting with transition metals in early and late barrier systems, with the aim of determining whether specific reaction parameter functionals for these systems might differ in a systematic way.
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