Pietro Cortona

An ab-initio study of the rôle of lone pairs in the structure and insulator–metal transition in SnO and PbO

We have performed density functional calculations on tetragonal SnO and PbO (litharge) in the space group P4/nmm with the specific intention of examining the role played by Sn 5s and Pb 6s lone pairs in stabilizing the structure, and in giving rise to semi-metallic behavior (of SnO at ambient pressure and of PbO in the γ phase). Use of the electron localization function has permitted real-space visualization of the lone pair in these structures. We also discuss the electronic structure of the orthorhombic PbO (massicot, space group Pbma) which again has localized lone pairs, contrary to some earlier expectation.

A new parameter-free correlation functional based on an average atomic reduced density gradient analysis

A new parameter-free correlation functional based on the local Ragot-Cortona approach [J. Chem. Phys. 121, 7671 (2004)] is presented. This functional rests on a single ansatz for the gradient correction enhancement factor: it is assumed to be given by a simple analytic expression satisfying some exact conditions and containing two coefficients. These coefficients are determined without implementing the functional and without using a fitting procedure to experimental data. Their values are determined by requiring that the functional gives a correct average reduced density gradient for atoms, which, to some extent, can be considered an intrinsic atomic property. The correlation functional is then coupled with the Perdew-Burke-Erzernhof (PBE) exchange and compared with the original PBE approach as well as with some other pure density or hybrid approaches. Standard tests for atomic and molecular systems show that our new functional significantly improves on PBE, showing very interesting properties.

Communication: One third: A new recipe for the PBE0 paradigm

We analyze the performances of the parameter-free hybrid density functional PBE0-1/3 obtained combining the PBE generalized-gradient functional with a predefined amount of exact exchange of 1/3, as recently discussed by Cortona [J. Chem. Phys. 136, 086101 (2012)]. The numerical results that we have obtained for various properties, such as atomization energies (G2-148 dataset), weak interactions (NCB31 dataset), hydrogen-bond length optimizations, and dissociation energies (HB10 dataset), and vertical excitation energies, show an increased performance of PBE0-1/3 with respect to the widely used PBE0. We therefore propose to use one third as the mixing coefficient for the PBE-based hybrid functional.

Ab initio calculations of cohesive and structural properties of the alkali-earth oxides

We have performed ab initio self-consistent calculations of the standard cohesive properties (equilibrium lattice parameters, binding energies and bulk moduli) of the four alkali-earth oxides (MgO, CaO, SrO and BaO) having the B1 (NaCl-like) phase under normal temperature and pressure conditions. We have also studied the relative stability of the B1 and B2 (CsCl-like) phases, and the behaviour, under pressure of these crystals (equations of state, transition pressures and changes of volume for the structural phase transition ). All the calculations were performed in the framework of the density-functional theory by a method which allows the direct calculation of the ground-state electron density of a system without the preliminary determination of its wavefunctions and energy eigenvalues.

Large thermal conductivity decrease in point defective Bi2Te3 bulk materials and superlattices

Defective Bi2Te3 structures have been studied with the aim of lowering the thermal conductivity in order to improve the thermoelectric figure of merit. The cross-plane thermal conductivities of structures containing point defects have been computed by means of molecular dynamics techniques, finding a maximum decrease of 70% for a 4% concentration of tellurium atom vacancies. Superlattices with modified stoichiometries have also been considered in order to find the configuration having the lowest thermal conductivity. In this case, a maximum decrease of 70% was also found. These predictions open the way to the design of efficient bulk thermoelectric materials having optimised thermal properties similar to those of superlattices.

Note: Theoretical mixing coefficients for hybrid functionals

The theoretical foundations of the double hybrid exchange-correlation functionals have been recently analyzed by Sharkas et al. [J. Chem. Phys. 134, 064113 (2011)10.1063/1.3544215] and, successively, by Bremond and Adamo [J. Chem. Phys. 135, 024106 (2011)10.1063/1.3604569] and by Toulouse et al. [J. Chem. Phys. 135, 101102 (2011)10.1063/1.3640019]. This analysis partially resulted in the introduction of a new class of double hybrids depending on just one parameter, the value of which was assumed to be 0.5 by Bremond and Adamo. In this note, I will suggest that other values can be chosen and that all these choices can be justified using the same theoretical arguments. These values are also “theoretical” mixing coefficients for single-hybrids, i.e., functional where only the exchange is hybridized.

Cohesive properties and behaviour under pressure of CaS, CaSe, and CaTe: results of ab initio calculations

We have performed ab initio calculations of CaS, CaSe and CaTe in the B1 (NaCl) and B2 (CsCl) phase. For each compound, we report results for the standard cohesive properties (equilibrium lattice parameters, bulk moduli, cohesive energies), for the equation of state and for the phase transition (transition pressure, compression ratio at the phase transition, percentage change of volume in the phase transition). A detailed comparison with recent experimental results is also performed.

Toward a Combined DFT/QTAIM Description of Agostic Bonds: The Critical Case of a Nb(III) Complex

A detailed analysis concerning the effect of the exchange-correlation functional on a prototypical agostic niobium complex has been carried out, with particular attention to a fundamental property of the functional, namely, the recovering of the uniform electron gas limit. The obtained results allow for revisiting the role of this limit for a proper description of the beta-H agostic interaction. Starting from these results, a new criterion for the bond analysis based on the electron density behavior is proposed. Indeed, the density homogeneity between the metal and the involved hydrogen has been evaluated at the bond critical point, as defined in the framework of Baders atoms in molecules theory, by calculating the average variation rates of the (reduced) density gradients. Such descriptors not only provide useful insights on the nature of such an interaction but also could be used as a starting point for a deep (and new) analysis of the chemical bond.

Semiclassical atom theory applied to solid-state physics

Using the semiclassical neutral atom theory, we extend to fourth order the modified gradient expansion of the exchange energy of density functional theory. This expansion can be applied both to large atoms and solid-state problems. Moreover, we show that it can be employed to construct a simple and nonempirical generalized gradient approximation (GGA) exchange-correlation functional competitive with state-of-the-art GGAs for solids, but also reasonably accurate for large atoms and ordinary chemistry.

Classical to Quantum Transition of Heat Transfer between Two Silica Clusters

Heat transfer between two silica clusters is investigated by using the nonequilibrium Greens function method. In the gap range between 4 Å and 3 times the cluster size, the thermal conductance decreases as predicted by the surface charge-charge interaction. Above 5 times the cluster size, the volume dipole-dipole interaction predominates. Finally, when the distance becomes smaller than 4 Å, a quantum interaction where the electrons of both clusters are shared takes place. This quantum interaction leads to the dramatic increase of the thermal coupling between neighbor clusters due to strong interactions. This study finally provides a description of the transition between radiation and heat conduction in gaps smaller than a few nanometers.