Francesco Casula

Cohesive Energies of Be and Mg Chalcogenides

We have calculated the cohesive energies, bulk moduli and equilibrium volumes of Be and Mg oxides, sulphides and selenides, in both zincblende and rocksalt structures. The calculations have been performed with the Discrete-Variational-Method (DVM), a real space first-principle local-density-functional approach. Comparisons with the experiment and with other first-principles approaches show that the electronic and structural properties of solids can be computed with DVM at least as accurately as with the usual plane-wave pseudopotential methods. This result is especially interesting in view of the fact that an order N implementation of DVM, based on the W. Yangs divide and conquer method, has been recently developed.

The SEISM project: a software engineering initiative for the study of materials

The present state of computational materials science is quite healthy and its future may be even better, as could be guessed by its excellent achievements and by the ever increasing number of dedicated workshops. Structured programming is no longer enough for dealing with the large software projects allowed by today`s computer hardware. An object-oriented computational model has been developed in order to achieve reuse, rapid prototyping and easy maintenance in large scale materials science calculations. The exclusive use of an object-oriented language is not mandatory for implementing the model. On the contrary, embedding Fortran code in an object-oriented language can be a very efficient way of fulfilling these goals without sacrificing the huge installed base of Fortran programs. Reuse can begin from one`s old Fortran programs. These claims are substantiated with practical examples from a professional code for the study of the electronic properties of atomic clusters. Out of the about 20,000 lines of the original Fortran program, more than 70% of them could be reused in the C++ objects of the new version. Facilities for dealing with periodic systems and for scaling linearly with the number of atoms have been added without any change in the computational model.

Electronic states and Schottky barriers at Pd2Si/Si(111) interfaces

Abstract It is possible to account for measured Pd/Si Schottky barrier heights (0.71-0.73 eV) by assuming that the interface between Si and Pd 2 Si includes a hexagonal Si transition region induced by the presence of Pd impurities. With the Fermi level pinned slightly above the Si valence band edge by hydridized PdSi states at the interface, as calculations indicate, the Schottky barrier height is determined primarily by the band gap of hexagonal Si, which is about 0.85 eV. Since Pd atoms are larger than Si atoms, isolated substitutional and interstitial Pd atoms are not readily incorporated into the Si lattice. We believe that the Pd impurities form triangular clusters at Si vacancies. Since these clusters are more easily accommodated in hexagonal than in cubic Si, the Si stacking sequence changes from cubic to hexagonal, giving rise to the transition region.

Classical versus ab initio structural relaxation: electronic excitations and optical properties of Ge nanocrystals embedded in an SiC matrix

We discuss and test a combined method to efficiently perform ground-state and excited-state calculations for relaxed structures using both a quantum first-principles approach and a classical molecular-dynamics scheme. We apply this method to calculate the ground state, the optical properties, and the electronic excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical molecular dynamics is used to relax the large-supercell system. First-principles quantum techniques are then used to calculate the electronic structure and, in turn, the electronic excitation and optical properties. The proposed procedure is tested with data resulting from a full first-principles scheme. The agreement is quantitatively discussed between the results after the two computational paths with respect to the structure, the optical properties, and the electronic excitations. The combined method is shown to be applicable to embedded nanocrystals in large simulation cells for which the first-principles treatment of the ionic relaxation is presently out of reach, whereas the electronic, optical and excitation properties can already be obtained ab initio. The errors incurred from the relaxed structure are found to be non-negligible but controllable.

Classical versus ab initio structural relaxation: electronic excitations and optical properties of Ge nanocrystals embedded in a SiC matrix

We discuss and test a combined method to efficiently perform ground- and excited-state calculations for relaxed structures using both a quantum first-principles approach and a classical molecular-dynamics scheme. We apply this method to calculate the ground state, the optical properties, and the electronic excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical molecular dynamics is used to relax the large-supercell system. First-principles quantum techniques are then used to calculate the electronic structure and, in turn, the electronic excitation and optical properties. The proposed procedure is tested with data resulting from a full first-principles scheme. The agreement is quantitatively discussed between the results after the two computational paths with respect to the structure, the optical properties, and the electronic excitations. The combined method is shown to be applicable to embedded nanocrystals in large simulation cells for which the first-principle treatment of the ionic relaxation is presently out of reach, whereas the electronic, optical and excitation properties can already be obtained ab initio . The errors incurred from the relaxed structure are found to be non-negligible but controllable.

Self-Energy Corrections to DFT-LDA Gaps of Realistic Carbon Nanotubes

Abstract : Since their discovery carbon nanotubes have attracted much interest for their peculiar electronic properties which go from metallic to semiconducting behaviour, depending both on diameter and chirality. The exact value of their band gap is obviously a crucial point to be addressed because it enters in the nanotube application as microelectronic devices. By making use of an efficient GW scheme, previously tested on bulk systems, as well as of a model screening function, we obtained for the first time excitation energies and band-gap values for carbon nanotubes. Results for (6, 0) and (7, 0) will be presented and discussed.