Rafael Ramirez

Strong covalent bonding between two graphene layers

Financed by CICYT under contracts MAT-2005-3866, MAT-2006-03741, FIS-2006-12117-C04-03,NAN-2004-09183-C10-08. We acknowledge the use of the Spanish Supercomputing Network and the CTI (CSIC).

Anomalous nuclear quantum effects in ice

One striking anomaly of water ice has been largely neglected and never explained. Replacing hydrogen (1H) by deuterium (2H) causes ice to expand, whereas the normal isotope effect is volume contraction with increased mass. Furthermore, the anomaly increases with temperature T, even though a normal isotope shift should decrease with T and vanish when T is high enough to use classical nuclear motions. In this study, we show that these effects are very well described by ab initio density-functional theory. Our theoretical modeling explains these anomalies, and allows us to predict and to experimentally confirm a counter effect, namely, that replacement of 16O by 18O causes a normal lattice contraction.

ELECTRON TRANSFER AND ELECTRONIC STRUCTURE OF KCUFES2

Abstract The electron transfer that occurs in the insertion of potassium in CuFeS 2 has been analyzed by two different techniques. The binding energies of the core electrons of CuFeS 2 , KCuFeS 2 and K x CuFeS 2 ( x ≈ 0) has been studied by XPS experiments, while the band structure of these phases has been investigated by tight-binding EH calculations. Both experimental and numerical results strongly support the belief that the insertion reaction induces a redox process in the starting CuFeS 2 , by which the Fe 3+ ions are reduced to Fe 2+ . Upon removal of the potassium ions, the Fe 2+ ions are oxidized to Fe 3+ . Copper atoms do not participate in the redox reaction.

Heat capacity of water : a signature of nuclear quantum effects

In this note we present results for the heat capacity at constant pressure for the TIP4PQ/2005 model, as obtained from path-integral simulations. The model does a rather good job of describing both the heat capacity of ice I(h) and of liquid water. Classical simulations using the TIP4P/2005, TIP3P, TIP4P, TIP4P-Ew, simple point charge/extended, and TIP5P models are unable to reproduce the heat capacity of water. Given that classical simulations do not satisfy the third law of thermodynamics, one would expect such a failure at low temperatures. However, it seems that for water, nuclear quantum effects influence the heat capacities all the way up to room temperature. The failure of classical simulations to reproduce C(p) points to the necessity of incorporating nuclear quantum effects to describe this property accurately.

Quantum contributions in the ice phases: The path to a new empirical model for water—TIP4PQ/2005

With a view to a better understanding of the influence of atomic quantum delocalization effects on the phase behavior of water, path integral simulations have been undertaken for almost all of the known ice phases using the TIP4P/2005 model in conjunction with the rigid rotor propagator proposed by Muser and Berne [Phys. Rev. Lett. 77, 2638 (1996)]. The quantum contributions then being known, a new empirical model of water is developed (TIP4PQ/2005) which reproduces, to a good degree, a number of the physical properties of the ice phases, for example, densities, structure, and relative stabilities.

The Schrödinger formulation of the Feynman path centroid density

We present an analysis of the Feynman path centroid density that provides new insight into the correspondence between the path integral and the Schrodinger formulations of statistical mechanics. The path centroid density is a central concept for several approximations (centroid molecular dynamics, quantum transition-state theory, and pure quantum self-consistent harmonic approximation) that are used in path integral studies of thermodynamic and dynamical properties of quantum particles. The centroid density is related to the quasistatic response of the equilibrium system to an external force. The path centroid dispersion is the canonical correlation of the position operator, which measures the linear change in the mean position of a quantum particle upon the application of a constant external force. At low temperatures, this quantity provides an approximation to the excitation energy of the quantum system. In the zero temperature limit, the particle’s probability density obtained by fixed centroid path in...

Path-integral simulation of solids

The path-integral formulation of the statistical mechanics of quantum many-body systems is described, with the purpose of introducing practical techniques for the simulation of solids. Monte Carlo and molecular dynamics methods for distinguishable quantum particles are presented, with particular attention to the isothermal-isobaric ensemble. Applications of these computational techniques to different types of solids are reviewed, including noble-gas solids (helium and heavier elements), group-IV materials (diamond and elemental semiconductors), and molecular solids (with emphasis on hydrogen and ice). Structural, vibrational, and thermodynamic properties of these materials are discussed. Applications also include point defects in solids (structure and diffusion), as well as nuclear quantum effects in solid surfaces and adsorbates. Different phenomena are discussed, as solid-to-solid and orientational phase transitions, rates of quantum processes, classical-to-quantum crossover, and various finite-temperature anharmonic effects (thermal expansion, isotopic effects, electron-phonon interactions). Nuclear quantum effects are most remarkable in the presence of light atoms, so that especial emphasis is laid on solids containing hydrogen as a constituent element or as an impurity.

Quantum path integral simulation of isotope effects in the melting temperature of ice Ih

The isotope effect in the melting temperature of ice Ih has been studied by free energy calculations within the path integral formulation of statistical mechanics. Free energy differences between isotopes are related to the dependence of their kinetic energy on the isotope mass. The water simulations were performed by using the q-TIP4P/F model, a point charge empirical potential that includes molecular flexibility and anharmonicity in the OH stretch of the water molecule. The reported melting temperature at ambient pressure of this model (T=251 K) increases by 6.5±0.5 and 8.2±0.5 K upon isotopic substitution of hydrogen by deuterium and tritium, respectively. These temperature shifts are larger than the experimental ones (3.8 and 4.5 K, respectively). In the classical limit, the melting temperature is nearly the same as that for tritiated ice. This unexpected behavior is rationalized by the coupling between intermolecular interactions and molecular flexibility. This coupling makes the kinetic energy of the OH stretching modes larger in the liquid than in the solid phase. However, the opposite behavior is found for intramolecular modes, which display larger kinetic energy in ice than in liquid water.

Quantum effects on the maximum in density of water as described by the TIP4PQ/2005 model.

Path integral simulations have been performed to determine the temperature of the maximum in density of water of the rigid, nonpolarizable TIP4PQ/2005 model treating long range Coulombic forces with the reaction field method. A maximum in density is found at 280 K, just 3 K above the experimental value. In tritiated water the maximum occurs at a temperature about 12 K higher than in water, in reasonable agreement with the experimental result. Contrary to the usual assumption that the maximum in classical water is about 14 K above that in water, we found that for TIP4PQ/2005 this maximum is about 30 K above. For rigid water models the internal energy and the temperature of maximum density do not follow a linear behavior when plotted as a function of the inverse of the hydrogen mass. In addition, it is shown that, when used with Ewald sums, the TIP4PQ/2005 reproduces quite nicely not only the maximum in density of water, but also the liquid densities, the structure of liquid water and the vaporization enthalpy. It was shown in a previous work that it also reproduces reasonably well the density and relative stabilities of ices. Therefore the TIP4PQ/2005 model, while still simple, allows one to analyze the interplay between quantum effects related to atomic masses and intermolecular forces in water.

Isotope effects in ice Ih: A path-integral simulation

Ice Ih has been studied by path-integral molecular dynamics simulations, using the effective q-TIP4P/F potential model for flexible water. This has allowed us to analyze finite-temperature quantum effects in this solid phase from 25 to 300 K at ambient pressure. Among these effects we find a negative thermal expansion of ice at low temperatures, which does not appear in classical molecular dynamics simulations. The compressibility derived from volume fluctuations gives results in line with experimental data. We have analyzed isotope effects in ice Ih by considering normal, heavy, and tritiated water. In particular, we studied the effect of changing the isotopic mass of hydrogen on the kinetic energy and atomic delocalization in the crystal as well as on structural properties such as interatomic distances and molar volume. For D(2)O ice Ih at 100 K we obtained a decrease in molar volume and intramolecular O-H distance of 0.6% and 0.4%, respectively, as compared to H(2)O ice.