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Showing papers by "Roberto Car published in 2011"


Journal ArticleDOI
TL;DR: The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice as discussed by the authors.
Abstract: The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated.

211 citations


Journal ArticleDOI
TL;DR: A novel theoretical approach for determining oxidation states (OS) from quantum-mechanical calculations for transition-metal ion systems is introduced using density-functional theory with a Hubbard U correction and the oxidation states were determined without ambiguity.
Abstract: We introduce a novel theoretical approach for determining oxidation states (OS) from quantum-mechanical calculations. For a transition-metal ion, for example, the metal–ligand orbital mixing contribution to the charge allocated to the ion is separated from that due to the actual occupation of the d-orbitals from which OS can then be inferred. We report the application of this approach to different transition-metal systems: molecular complexes, ruthenium-dye molecules, ruthenium complexes with noninnocent ligands, and bulk semiconductors. The computations were carried out using density-functional theory with a Hubbard U correction. The oxidation states were determined without ambiguity.

101 citations


Journal ArticleDOI
TL;DR: In this paper, the quantum nature of the protons participating in hydrogen bonds in several ice structures was analyzed by analyzing the one particle density matrix. And the correlations appeared along the path to hydrogen bond symmetrization, when quantum fluctuations delocalize the proton on the two bond sides.
Abstract: We study the quantum nature of the protons participating in hydrogen bonds in several ice structures by analyzing the one particle density matrix. We find that in all cases, including ice Ih, the most common form of ice, and the high pressure phases, ice VIII, VII, and X, the system is ground-state dominated. However, while the dynamics is uncorrelated in the structures with standard asymmetric hydrogen bonds, such as ice Ih and VIII, local correlations among the protons characterize ice VII and, to a lesser extent, ice X in the so-called low barrier hydrogen bond regime. The correlations appear along the path to hydrogen bond symmetrization, when quantum fluctuations delocalize the proton on the two bond sides. The correlations derive from a strong requirement for local charge neutrality that favors concerted motion along the bonds. The resulting behavior deviates substantially from mean field theory, which would predict in ice VII coherent tunneling of the proton between the two bond sides, thereby causing an ionization catastrophe. Due to the correlations, the quantum state of the proton is entangled.

56 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the protons in hexagonal ice experience an anisotropic quasiharmonic effective potential with three distinct principal frequencies that reflect molecular orientation.
Abstract: By analyzing the momentum distribution obtained from path integral and phonon calculations we find that the protons in hexagonal ice experience an anisotropic quasiharmonic effective potential with three distinct principal frequencies that reflect molecular orientation. Due to the importance of anisotropy, anharmonic features of the environment cannot be extracted from existing experimental distributions that involve the spherical average. The full directional distribution is required, and we give a theoretical prediction for this quantity that could be verified in future experiments. Within the quasiharmonic context, anharmonicity in the ground-state dynamics of the proton is substantial and has quantal origin, a finding that impacts the interpretation of several spectroscopies.

34 citations


Journal ArticleDOI
TL;DR: This paper analyzes two cases, superoxide reductase and a proposed hydrogen-producing model electrocatalyst [FeS(2)]/[FeFe](P), a modification of the active site of the diiron hydrogenase enzymes, and introduces a new, generally useful local method for determining OSs, their changes, and the associated bonding changes and electron flow.
Abstract: In catalysis by metalloenzymes and in electrocatalysis by clusters related in structure and composition to the active components of such enzymes transition-metal atoms can play a central role in the catalyzed redox reactions. Changes to their oxidation states (OSs) are critical for understanding the reactions. The OS is a local property and we introduce a new, generally useful local method for determining OSs, their changes, and the associated bonding changes and electron flow. The method is based on computing optimally localized orbitals (OLOs). With this method, we analyze two cases, superoxide reductase (SOR) and a proposed hydrogen-producing model electrocatalyst [FeS(2)]/[FeFe](P), a modification of the active site of the diiron hydrogenase enzymes. Both utilize an under-coordinated Fe site where a one-electron reduction (for SOR) or a two-electron reduction (for [FeFe](P)) of the substrate occurs. We obtain the oxidation states of the Fe atoms and of their critical ligands, the changes of the bonds to those ligands, and the electron flow during the catalytic cycle, thereby demonstrating that OLOs constitute a powerful interpretive tool for unraveling reaction mechanisms by first-principles computations.

29 citations


Journal ArticleDOI
TL;DR: In this article, the authors simulate the behavior of model clusters by first-principles electronic-structure and molecular-dynamics simulations, and then design a successful phosphorous-substituted [FeFe]P cluster.

6 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of interfacial disorder on the average spin transfer torque in magnetic tunnel junctions (MTJ) was studied and generalized the nonequilibrium Green's functions Keldysh formalism.
Abstract: We have generalized the nonequilibrium Green’s functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, 〈T∥〉, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average 〈T∥〉 as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average 〈T∥〉 in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of 〈T∥〉 in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.

3 citations