Giuseppe Mallia

Periodic quantum mechanical simulation of the He-MgO(100) interaction potential

He-atom scattering is a well established and valuable tool for investigating surface structure. The correct interpretation of the experimental data requires an accurate description of the He-surface interaction potential. A quantum-mechanical treatment of the interaction potential is presented using the current dominant methodologies for computing ground state energies (Hartree-Fock, local and hybrid-exchange density functional theory) and also a novel post-Hartree-Fock ab initio technique for periodic systems (a local implementation of Mo̸ller-Plesset perturbation theory at second order). The predicted adsorption well depth and long range behavior of the interaction are compared with that deduced from experimental data in order to assess the accuracy of the interaction potential.

Parallel implementation of the ab initio CRYSTAL program: electronic structure calculations for periodic systems

CRYSTAL is an ab initio electronic structure program, based on the linear combination of atomic orbitals, for periodic systems. This paper concerns the ability of CRYSTAL to exploit massively parallel computer hardware. A brief review of the theory, numerical implementations and parallel solutions will be given and some of the functionalities and capabilities highlighted. Some features that are unique to CRYSTAL will be described and development plans outlined.

Chemistry of defect induced photoluminescence in chalcopyrites: The case of CuAlS2

Hybrid exchange density functional theory is used to study the wide band gap chalcopyrite CuAlS2. The formation energies of charged and neutral intrinsic defects are calculated for different environmental conditions, and it is shown that CuAlS2 is a p-type material that cannot be type inverted through the formation of intrinsic defects. The calculated band gap states associated with the different intrinsic defects are used to comment on the origin of the observed CuAlS2 photoluminescence emissions. The origin and stability of ordered defect compounds derived from CuAlS2 are investigated, and it is concluded that CuAl5S8 is a stable ordered defect compound, albeit in a small region of phase space.

Ab initio complex band structure of conjugated polymers: Effects of hydrid density functional theory and GW schemes

The non-resonant tunneling regime for charge transfer across nanojunctions is critically dependent on the so-called \beta{} parameter, governing the exponential decay of the current as the length of the junction increases. For periodic materials, this parameter can be theoretically evaluated by computing the complex band structure (CBS) -- or evanescent states -- of the material forming the tunneling junction. In this work we present the calculation of the CBS for organic polymers using a variety of computational schemes, including standard local, semilocal, and hybrid-exchange density functionals, and many-body perturbation theory within the GW approximation. We compare the description of localization and \beta{} parameters among the adopted methods and with experimental data. We show that local and semilocal density functionals systematically underestimate the \beta{} parameter, while hybrid-exchange schemes partially correct for this discrepancy, resulting in a much better agreement with GW calculations and experiments. Self-consistency effects and self-energy representation issues of the GW corrections are discussed together with the use of Wannier functions to interpolate the electronic band-structure.

The stability of LaMnO3 surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst

LaMnO3 is an inexpensive alternative to precious metals (e.g. platinum) as a catalyst for the oxygen reduction reaction in alkaline fuel cells. In fact, recent studies have shown that among a range of non-noble metal catalysts, LaMnO3 provides the highest catalytic activity. Despite this, very little is known about LaMnO3 in the alkaline fuel cells environment, where the orthorhombic structure is most stable. In order to understand the reactivity of orthorhombic LaMnO3 we must first understand the surface structure. Hence, we have carried out calculations on its electrostatically stable low index surfaces using hybrid-exchange density functional theory, as implemented in CRYSTAL09. For each surface studied the calculated structure and formation energy is discussed. Among the surfaces studied the (100) surface was found to be the most stable with a formation energy of 0.98 J m−2. The surface energies are rationalised in terms of the cleavage of Jahn–Teller distorted Mn–O bonds, the compensation of undercoordination for ions in the terminating layer and relaxation effects. Finally, the equilibrium morphology of orthorhombic LaMnO3 crystals is predicted, allowing us to speculate about likely surface reaction sites.

An alternative approach for the calculation of correlation energy in periodic systems: a hybrid MP2(B3LYP) study of the He-MgO(100) interaction.

A practical and efficient method for exploiting second order Rayleigh-Schrödinger perturbation theory to approximate the correlation energy contribution to the London dispersion interaction is presented. The correlation energy is estimated as the Møller-Plesset contribution computed using single particle orbitals from hybrid exchange density functional theory as the reference state.

A hybrid-exchange density functional study of the bonding and electronic structure in bulk CuFeS2

The geometric, electronic, and magnetic properties of bulk chalcopyrite CuFeS2 have been investigated using hybrid-exchange density functional theory calculations. The results are compared with available theoretical and experimental data. The theoretical description of the bonding and electronic structure in CuFeS2 is analyzed in detail and compared to those computed for chalcocite (CuS2) and greigite (Fe3S4). In particular, the behavior of the 3d electrons of Fe(3+) is discussed in terms of the Hubbard-Anderson model in the strongly correlated regime and found to be similarly described in both materials by an on-site Coulomb repulsion (U) of ∼8.9 eV and a transfer integral (t) of ∼0.3 eV.

Calculating charged defects using CRYSTAL

The methodology for the calculation of charged defects using the CRYSTAL program is discussed. Two example calculations are used to illustrate the methodology: He+ ions in a vacuum and two intrinsic charged defects, Cu vacancies and Ga substitution for Cu, in the chalcopyrite CuGaS2.

A combined thermodynamics and first principles study of the electronic, lattice and magnetic contributions to the magnetocaloric effect in La0.75Ca0.25MnO3

Manganites with the formula La1−x Ca x MnO3 for 0.2 < x < 0.5 undergo a magnetic field driven transition from a paramagnetic to ferromagnetic state, which is accompanied by changes in the lattice and electronic structure. An isotropic expansion of the La0.75Ca0.25MnO3 cell at the phase transition has been observed experimentally. It is expected that there will be a large entropy change at the transition due to its first order nature. Doped lanthanum manganite (LMO) is therefore of interest as the active component in a magnetocaloric cooling device. However, the maximum obtained value for the entropy change in Ca-doped manganites merely reaches a moderate value in the field of a permanent magnet. The present theoretical work aims to shed light on this discrepancy. A combination of finite temperature statistical mechanics and first principles theory is applied to determine individual contributions to the total entropy change of the system by treating the electronic, lattice and magnetic components independently. Hybrid-exchange density functional (B3LYP) calculations and Monte Carlo simulations are performed for La0.75Ca0.25MnO3. Through the analysis of individual entropy contributions, it is found that the electronic and lattice entropy changes oppose the magnetic entropy change. The results highlighted in the present work demonstrate how the electronic and vibrational entropy contributions can have a deleterious effect on the total entropy change and thus the potential cooling power of doped LMO in a magnetocaloric device.

A hybrid-exchange density functional study of Ca-doped LaMnO3

In this work, hybrid-exchange density functional theory calculations are carried out to determine the effects of A-site doping on the electronic and magnetic properties of the manganite series La1−xCaxMnO3. This study focuses on the nature of the ground state for an ordered Ca distribution. We show that the hybrid exchange functional, B3LYP, provides an accurate and consistent description of the electronic structure for LaMnO3 and La0.75Ca0.25MnO3. The magnetic ground states for the compositions studied are predicted correctly and comparisons have been made to available experimental data. This provides a basis for a first principles description of the magnetocaloric effect in La1−xCaxMnO3.