Jorge Kohanoff

Ab initio simulation of charged slabs at constant chemical potential

We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential μ, rather than at constant number of electrons Ne. To this end, we define the chemical potential relative to a plane (or “reference electrode”) at a finite distance from the slab (the distance should reflect the particular geometry of the situation being modeled). To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The “constant μ” mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry.

A possible new highly stable fulleride cluster: Li12C60

Abstract Recent electrochemical evidence appears to indicate that in the solid state it is possible to obtain a stable compound of stoichiometry Li12C60. Prompted by these findings we have investigated by ab initio molecular dynamics the possibility that Li12C60 might exist as a stable cluster. Our results show that a structure with Ih symmetry is stable, which also corresponds to a HOMO-LUMO gap of the same order as that of C60. The presence of the Li atoms induces a polarization of the electronic states of the cage, and gives rise to a significant weakening of the double bonds. From a molecular dynamics simulation we compute the frequencies of the optically active vibrational modes.

Simulation of interfaces between room temperature ionic liquids and other liquids

The structure and properties of the interfaces between the room temperature ionic liquid dimethylimidazolium chloride ([dmim]Cl) and different Lennard-Jones fluids and between ionic liquid and water have been studied by molecular dynamics simulations, and compared to the ionic liquid-vapour interface. Two contrasting types of interface were investigated, thermodynamically stable interfaces between ionic liquid and vapour and between ionic liquid and Lennard-Jones fluids, and diffusing interfaces between miscible phases of different compositions involving water. The density profiles of different species through the interface are presented. The cations and water molecules near the former type of interface are aligned relative to the surface, but no orientational preference was found near or in the broad diffusing interface. The ionic liquid has a negative electrostatic potential relative to vapour or Lennard-Jones fluid, but is more positive than pure water. This contrast is explained in terms of the relative importance of orientation and concentration differences in the two types of interface.

Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals.

Based on an accurate first principles description of the energetics in H-bonded potassium-dihydrogen-phosphate crystals, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Tunneling is allowed only for clusters involving correlated protons and heavy ion displacements, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples self-consistently with the proton off-centering, thus explaining both the giant isotope effect and its close connection with geometrical effects.

The puzzling stability of monatomic gold wires

We have examined theoretically the spontaneous thinning process of tip-suspended nanowires, and subsequently studied the structure and stability of the monatomic gold wires recently observed by transmission electron microscopy. The methods used include thermodynamics, classical many-body force simulations, local density and generalized gradient electronic structure calculations as well as ab initio simulations including the two tips. The wire thinning is well explained in terms of a thermodynamic tip suction driving migration of surface atoms from the wire to the tips. For the same reason the monatomic wire becomes progressively stretched. Surprisingly, however, all calculations so far indicate that the stretched monatomic gold wire should be unstable against breaking, contrary to the apparent experimental stability. The possible reasons for this stability are discussed.

First-principles study of ferroelectricity and isotope effects in H-bonded KH2PO4 crystals

By means of extensive first-principles calculations we studied the ferroelectric phase transition and the associated isotope effect in

Phonon spectra from short non-thermally equilibrated molecular dynamics simulations

\mathrm{K}{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}

Hybrid quantum and classical mechanical Monte Carlo simulations of the interaction of hydrogen chloride with solid water clusters

(KDP). Our calculations revealed that the spontaneous polarization of the ferroelectric phase is due to electronic charge redistributions and ionic displacements which are a consequence of proton ordering, and not vice versa. The experimentally observed double-peaked proton distribution in the paraelectric phase cannot be explained by a dynamics of only protons. This requires, instead, collective displacements within clusters that include also the heavier ions. These tunneling clusters can explain the recent evidence of tunneling obtained from Compton scattering measurements. The sole effect of mass change upon deuteration is not sufficient to explain the huge isotope effect. Instead, we find that structural modifications deeply connected with the chemistry of the H bonds produce a feedback effect on tunneling that strongly enhances the phenomenon. The resulting influence of the geometric changes on the isotope effect agrees with experimental data from neutron scattering. Calculations under pressure allowed us to analyze the issue of universality in the disappearance of ferroelectricity upon compression. Compressing DKDP so that the distance between the two peaks in the deuteron distribution is the same as for protons in KDP, corresponds to a modification of the underlying double-well potential, which becomes

Solid Molecular Hydrogen: The Broken Symmetry Phase

23\phantom{\rule{0.3em}{0ex}}\mathrm{meV}

Ab initio molecular dynamics of C70. Intramolecular vibrations and zero-point motion effects

shallower. This energy difference is what is required to modify the