Showing papers by "Daniel Sánchez-Portal published in 2001"

Numerical atomic orbitals for linear-scaling calculations

Abstract: The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. Strict localization is maintained while ensuring the continuity of the basis-function derivative at the cutoff radius. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic, and metallic. Satisfactory convergence is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.

The SIESTA method for ab initio order-N materials simulation

Abstract: We have developed and implemented a self-consistent density functional method using standard norm-conserving pseudopotentials and a flexible, numerical LCAO basis set, which includes multiple-zeta and polarization orbitals. Exchange and correlation are treated with the local spin density or generalized gradient approximations. The basis functions and the electron density are projected on a real-space grid, in order to calculate the Hartree and exchange-correlation potentials and matrix elements, with a number of operations that scales linearly with the size of the system. We use a modified energy functional, whose minimization produces orthogonal wavefunctions and the same energy and density as the Kohn-Sham energy functional, without the need of an explicit orthogonalization. Additionally, using localized Wannier-like electron wavefunctions allows the computation time and memory, required to minimize the energy, to also scale linearly with the size of the system. Forces and stresses are also calculated efficiently and accurately, thus allowing structural relaxation and molecular dynamics simulations.

Linear Scaling DFT Calculations with Numerical Atomic Orbitals

Abstract: We have recently developed a method to perform Density Functional Theory calculations in systems with a very large number of atoms, which is based on the use of numerical atomic orbitals as basis sets. The method incorporates Order-N techniques both in the calculation of the Kohn-Sham hamiltonian matrix elements and in the solution of the wave functions, which make the CPU time and memory to scale linearly with the number of atoms, allowing calculations in very large system. In this work, we present results on several test systems to show that the approach and the basis sets used with our method are able to provide an accuracy similar to that of other standard DFT techniques.

Hybrid DNA-gold nanostructured materials: an ab initio approach

Abstract: The controlled assembly of metal nanoparticles into macroscopic materials using DNA oligonucleotides has opened new directions of research in nanoscience and nanotechnology. Here, we describe recent ab initio calculations on structural and electronic properties of the subsystems forming these materials: bare and thiol-passivated gold nanoclusters, gold nanowires and fragments of DNA chains. Our results indicate that gold nanoclusters are distorted dramatically by a passivating methylthiol monolayer, that monatomic gold chains are stable in zigzag geometries and that dry acidic λ-DNA is a good insulator. These results provide useful insights towards the complete understanding, design and proper utilization of hybrid DNA-gold nanostructured materials.

Interplay between theory and experiment in solid state inorganic chemistry

Abstract: Recent advances in both first principles computational methodologies for complex systems and qualitative understanding of the electronic structure of solids now make a real dialogue between theoreticians and experimentalists possible. We discuss how this situation can lead to a fruitful interplay in inorganic solid state chemistry, considering selected examples, such as the structure of gold nanoclusters and nanowires, the absorption of atoms and molecules and growth of thin films on silicon surfaces, understanding the development of some structural modulations in low-dimensional transition metal oxides and bronzes, and the control of the transport properties of hybrid organic–inorganic charge transfer molecular solids.

LCAO calculation of dynamical charges and ferroelectricity

Abstract: In this paper we discuss some of the details of the calculation of the macroscopic polarization, via the Berry-phase approach, using a general (numerical) local combination of atomic orbitals (LCAO) as a basis set. Previous implementations with a LCAO basis relied on the use of Gaussian expansions, where analytical formulas exist for the crucial matrix elements. However, our approach, which only requires the matrix elements of the position operator, can be more easily implemented in the case of numerical orbitals. This work is a necessary first step towards our main goal of applying the SIESTA code to the study of ferroelectric alloys. This code, which uses a numerical LCAO basis set, has been highly optimized for the treatment of large systems. We are confident that it will allow us to treat larger systems than the standard plane-waves methods, while keeping a reasonable accuracy. Test results for representative ferroelectric perovskites are presented.