6,626 research outputs found
Can optical spectroscopy directly elucidate the ground state of C20?
The optical response of the lowest energy members of the C20 family is
calculated using time-dependent density functional theory within a real-space,
real-time scheme. Significant differences are found among the spectra of the
different isomers, and thus we propose optical spectroscopy as a tool for
experimental investigation of the structure of these important clusters.Comment: 11 pages, 2 figures. To be published in J. Chem. Phy
On the Combination of TDDFT with Molecular Dynamics: New Developments
In principle, we should not need the time-dependent extension of
density-functional theory (TDDFT) for excitations, and in particular not for
Molecular Dynamics (MD) studies: the theorem by Hohenberg and Kohn teaches us
that for any observable that we wish to look at (including dynamical properties
or observables dependent on excited states) there is a corresponding functional
of the ground-state density. Yet the unavailability of such magic functionals
in many cases (the theorem is a non-constructive existence result) demands the
development and use of the alternative exact reformulation of quantum mechanics
provided by TDDFT. This theory defines a convenient route to electronic
excitations and to the dynamics of a many-electron system subject to an
arbitrary time-dependent perturbation. This is, in fact, the main purpose of
inscribing TDDFT in a MD framework -the inclusion of the effect of electronic
excited states in the dynamics. However, as we will show in this review, it may
not be the only use of TDDFT in this context. In this manuscript, we review two
recent proposals: In Section 1.2, we show how TDDFT can be used to design
efficient gsBOMD algorithms -even if the electronic excited states are in this
case not relevant. The work described in Section 1.3 addresses the problem of
mixed quantum-classical systems at thermal equilibrium.Comment: 10 pages, 1 figure, to be published in the book "Time Dependent
Density Functional Theory" by Springer Verla
Nonextensive thermodynamic functions in the Schr\"odinger-Gibbs ensemble
Schr\"odinger suggested that thermodynamical functions cannot be based on the
gratuitous allegation that quantum-mechanical levels (typically the orthogonal
eigenstates of the Hamiltonian operator) are the only allowed states for a
quantum system [E. Schr\"odinger, Statistical Thermodynamics (Courier Dover,
Mineola, 1967)]. Different authors have interpreted this statement by
introducing density distributions on the space of quantum pure states with
weights obtained as functions of the expectation value of the Hamiltonian of
the system.
In this work we focus on one of the best known of these distributions, and we
prove that, when considered in composite quantum systems, it defines partition
functions that do not factorize as products of partition functions of the
noninteracting subsystems, even in the thermodynamical regime. This implies
that it is not possible to define extensive thermodynamical magnitudes such as
the free energy, the internal energy or the thermodynamic entropy by using
these models. Therefore, we conclude that this distribution inspired by
Schr\"odinger's idea can not be used to construct an appropriate quantum
equilibrium thermodynamics.Comment: 32 pages, revtex 4.1 preprint style, 5 figures. Published version
with several changes with respect to v2 in text and reference
Bioorthogonal catalytic activation of Platinum and Ruthenium anticancer complexes by FAD and flavoproteins
Recent advances in bioorthogonal catalysis promise to deliver new chemical tools for performing chemoselective transformations in complex biological environments. Herein, we report how FAD (flavin adenine dinucleotide), FMN (flavin mononucleotide), and four flavoproteins act as unconventional photocatalysts capable of converting PtIV and RuII complexes into potentially toxic PtII or RuII-OH2 species. In the presence of electron donors and low doses of visible light, the flavoproteins mini singlet oxygen generator (miniSOG) and NADH oxidase (NOX) catalytically activate PtIV prodrugs with bioorthogonal selectivity. In the presence of NADH, NOX catalyzes PtIV activation in the dark as well, indicating for the first time that flavoenzymes may contribute to initiating the activity of PtIV chemotherapeutic agents
Tissue factor as a novel marker for detection of circulating cancer cells
Tissue factor (TF) is a molecular marker that is up-regulated in cancer cells and aids tumoral dissemination. Our purpose was to develop a nested RT-PCR strategy against TF for detecting blood-borne tumour cells. Our method detected TF expression in a minimum of 1.5 pg total RNA from MCF7 cells. A preliminary study in blood samples from 16 advanced breast carcinoma patients showed that 80% of patients with high TF load progressed and died, while only 18% with low TF load showed the same behaviour. Kaplan-Meier analysis confirmed worse overall survival in patients with high TF load.Fil: Otero, L. L.. Universidad Nacional de Quilmes; ArgentinaFil: Alonso, Daniel Fernando. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Castro, M.. Universidad de Buenos Aires; ArgentinaFil: Cinat, G.. Universidad de Buenos Aires; ArgentinaFil: Gabri, M. R.. Universidad Nacional de Quilmes; ArgentinaFil: Gomez, Daniel Eduardo. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentin
About the computation of finite temperature ensemble averages of hybrid quantum-classical systems with molecular dynamics
Molecular or condensed matter systems are often well approximated by hybrid quantum-classical models: the electrons retain their quantum character, whereas the ions are considered to be classical particles. We discuss various alternative approaches for the computation of equilibrium (canonical) ensemble averages for observables of these hybrid quantum-classical systems through the use of molecular dynamics (MD)-i.e. by performing dynamics in the presence of a thermostat and computing time-averages over the trajectories. Often, in classical or ab initio MD, the temperature of the electrons is ignored and they are assumed to remain at the instantaneous ground state given by each ionic configuration during the evolution. Here, however, we discuss the general case that considers both classical and quantum subsystems at finite temperature canonical equilibrium. Inspired by a recent formal derivation for the canonical ensemble for quantum classical hybrids, we discuss previous approaches found in the literature, and provide some new formulas
Entropy and canonical ensemble of hybrid quantum classical systems
We generalize von Neumann entropy function to hybrid quantum-classical
systems by considering the principle of exclusivity of hybrid events. For
non-interacting quantum and classical subsystems, this entropy function
separates into the sum of the usual classical (Gibbs) and quantum (von Neumann)
entropies, whereas if the two parts do interact, it can be properly separated
into the classical entropy for the marginal classical probability, and the
conditional quantum entropy.
We also deduce the hybrid canonical ensemble (HCE) as the one that maximizes
this entropy function, for a fixed ensemble energy average. We prove that the
HCE is additive for non-interacting systems for all thermodynamic magnitudes,
and reproduces the appropriate classical- and quantum-limit ensembles.
Furthermore, we discuss how and why Ehrenfest dynamics does not preserve the
HCE and does not yield the correct ensemble averages when time-averages of
simulations are considered -- even if it can still be used to obtain correct
averages by modifying the averaging procedure.Comment: 6 pages + 4 pages Supp. Ma
The interplay between double exchange, super-exchange, and Lifshitz localization in doped manganites
Considering the disorder caused in manganites by the substitution of Mn by Fe
or Ga, we accomplish a systematic study of doped manganites begun in previous
papers. To this end, a disordered model is formulated and solved using the
Variational Mean Field technique. The subtle interplay between double exchange,
super-exchange, and disorder causes similar effects on the dependence of T_C on
the percentage of Mn substitution in the cases considered. Yet, in
LaCaMnGaO our results suggest a quantum
critical point (QCP) for , associated to the localization of
the electronic states of the conduction band. In the case of
LaCaMnFeO (with ) no such QCP is expected.Comment: 6 pages + 3 postscript figures. Largely extended discussio
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