19 research outputs found
Stacking of oligo and polythiophenes cations in solution: surface tension and dielectric saturation
The stacking of positively charged (or doped) terthiophene oligomers and
quaterthiophene polymers in solution is investigated applying a recently
developed unified electrostatic and cavitation model for first-principles
calculations in a continuum solvent. The thermodynamic and structural patterns
of the dimerization are explored in different solvents, and the distinctive
roles of polarity and surface tension are characterized and analyzed.
Interestingly, we discover a saturation in the stabilization effect of the
dielectric screening that takes place at rather small values of .
Moreover, we address the interactions in trimers of terthiophene cations, with
the aim of generalizing the results obtained for the dimers to the case of
higher-order stacks and nanoaggregates
Simulation of Heme using DFT+U: a step toward accurate spin-state energetics
We investigate the DFT+U approach as a viable solution to describe the
low-lying states of ligated and unligated iron heme complexes. Besides their
central role in organometallic chemistry, these compounds represent a
paradigmatic case where LDA, GGA, and common hybrid functionals fail to
reproduce the experimental magnetic splittings. In particular, the imidazole
pentacoordinated heme is incorrectly described as a triplet by all usual DFT
flavors. In this study we show that a U parameter close to 4 eV leads to spin
transitions and molecular geometries in quantitative agreement with
experiments, and that DFT+U represents an appealing tool in the description of
iron porphyrin complexes, at a much reduced cost compared to correlated
quantum-chemistry methods. The possibility of obtaining the U parameter from
first-principles is explored through a self-consistent linear-response
formulation. We find that this approach, which proved to be successful in other
iron systems, produces in this case some overestimation with respect to the
optimal values of U.Comment: To be published in The Journal of Physical Chemistry B 30 pages, 15
figure
Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
Transition-metal centers are the active sites for many biological and
inorganic chemical reactions. Notwithstanding this central importance,
density-functional theory calculations based on generalized-gradient
approximations often fail to describe energetics, multiplet structures,
reaction barriers, and geometries around the active sites. We suggest here an
alternative approach, derived from the Hubbard U correction to solid-state
problems, that provides an excellent agreement with correlated-electron quantum
chemistry calculations in test cases that range from the ground state of Fe
and Fe to the addition-elimination of molecular hydrogen on FeO. The
Hubbard U is determined with a novel self-consistent procedure based on a
linear-response approach.Comment: 5 pages, 3 figures, Phys. Rev. Lett., in pres
The magnetic structure of β-cobalt hydroxide and the effect of spin-orientation
Synchrotron X-ray and neutron diffraction experiments at various temperatures, down to 3 K, along with ab initio calculations, are carried out to elucidate the magnetic order of layered β-cobalt-hydroxide. This combination of techniques allows for the unambiguous assignment of the magnetic structure of this material. Our results confirm that below the Néel temperature high-spin cobalt centers are ferromagnetically coupled within a layer, and antiferromagnetically coupled across layers (magnetic propagation vector k = (0,0,½)), in agreement with the indirect interpretation based on magnetic susceptibility measurements. A paramagnetic/antiferromagnetic transition is observed at around 15 K. Moreover, the thermal expansion behavior along the c-lattice direction, perpendicular to the layers, shows an inflection slightly above this temperature, at around 30 K. The neutron diffraction patterns and the non-collinear DFT+U calculations indicate that the magnetization forms an angle of about 35° with the cobalt planes. In particular, for an isolated ferromagnetic layer, the electronic structure calculations reveal sharp cusps on the potential energy surface when the spins point parallel or perpendicular to the planes, suggesting that the ferromagnetic superexchange mechanism is strongly sensitive to the orientation of the magnetic moment.JARV acknowledges CSIC for a JAEdoc contract. This study has been supported by grants of ANPCYT/PICT 2012-2292 and UBACYT 20020120100333BA.Peer Reviewe
Dissociation free energy profiles for water and methanol on TiO2 surfaces
The umbrella sampling methodology is applied in the framework of density functional theory and Car-Parrinello molecular dynamics simulations to obtain the free energy profiles for the dissociation of methanol and water on stoichiometric TiO2 surfaces. In particular, we study the dissociation of water on rutile (110) and anatase (101), and the dissociation of the O-H and C-O bonds of methanol on anatase (101). We discuss the reaction free energies and activation barriers of these processes in the light of experiments and previous simulations at zero temperature. The entropic contribution to the reaction free energy is found to be positive for the dissociation of water and negative for the dissociation of methanol. © 2010 American Chemical Society.Fil: Sanchez, Veronica Muriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; ArgentinaFil: Cojulun, Julie A.. University of California at Irvine; Estados UnidosFil: Scherlis Perel, Damian Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentin
Radiative thermalization in semiclassical simulations of light-matter interaction
Prediction of the equilibrium populations in quantum dynamics simulations of molecules exposed to black-body radiation has proved challenging for semiclassical treatments, with the usual Ehrenfest and Maxwell-Bloch methods exhibiting serious failures. In this context, we explore the behavior of a recently introduced semiclassical model of light-matter interaction derived from a dissipative Lagrangian [C. M. Bustamante, E. D. Gadea, A. Horsfield, T. N. Todorov, M. C. González Lebrero, and D. A. Scherlis, Phys. Rev. Lett. 126, 087401 (2021)0031-900710.1103/PhysRevLett.126.087401]. It is shown that this model reproduces the Boltzmann populations for two-level systems, predicting the black-body spectra in approximate agreement with Planck's distribution. In multilevel systems, small deviations from the expected occupations are seen beyond the first excited level. By averaging over fast oscillations, a rate equation is derived from the dissipative equation of motion that makes it possible to rationalize these deviations. Importantly, it enables us to conclude that this model will produce the correct equilibrium populations provided the occupations of the lowest levels remain close to unity, a condition satisfied at low temperature or small excitations.Fil: Gadea, Esteban David. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Bustamante, Carlos Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Todorov, Tchavdar N.. The Queens University of Belfast; IrlandaFil: Scherlis Perel, Damian Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin