My research has been mainly devoted to time-dependent DFT (TDDFT). TDDFT is a reliable and precise tool for the calculation of photoabsorption spectra of finite systems. Furthermore, it can handle nonlinear response, like the interaction between a molecule and a strong laser field. My work in this field has three main components:
1. I am the main author of the computer code octopus, and the leader of the current development team. This program, which is open source software under the GNU general public license (GPL), simulates the dynamics of electrons and nuclei under the influence of time-dependent fields. The electronic degrees of freedom are treated quantum-mechanically within TDDFT, while the nuclei are considered to behave as classical point particles. In this code, all quantities are discretized in real space using a uniform grid, and the simulations are performed in real time. Over the past years, octopus has evolved into a fairly complex and complete tool, and is now used by several research groups around the world. Due to the open nature of the project, it is hard to estimate the total number of users. However, an estimate can be made from the number of downloads (more than 300 just in December 2003), and from the number of participants in the users mailing list (more than 40).
2. I have employed octopus to study several systems, like small molecules, fullerenes, etc. Some of these works regarded the calculation of photoabsorption spectra, while others treated the nonlinear response of molecules under the influence of ultrashort laser fields. Among these, I would like to emphasize my work on the green fluorescent protein (GFP). In this work, QM/MM techniques were combined with TDDFT to calculate the photoabsorption spectra of the GFP. The calculated spectra were in excellent agreement with available experimental data. Furthermore, it was possible to estimate that the two forms of the GFP photoreceptor appear in vivo in a ~1/4 ratio, which was confirmed by experimental evidences. 3. Furthermore, I worked on the generalization of the electron localization function (ELF). This function can be used to analyze time-dependent processes. The time-dependent ELF allows the timeresolved observation of the formation, the modulation, and the breaking of chemical bonds, and can thus provide a visual understanding of complex reactions involving the dynamics of excited electrons. The usefulness of the time-dependent ELF was illustrated by two examples: the Pi-Pi* transition induced by a laser field, and the destruction of bonds and formation of lone pairs in a scattering process.
More recently I have been interested in the mechanical properties of nanotubes under stress. My main motivation were experimental TEM studies of the breaking process of single-wall carbon nanotubes. Depending on the conditions, they observed brittle or ductile behavior. In this last case, they sometimes observed the formation of long chains of carbon atoms. In order to understand these results, I performed extensive tightbinding simulations of the breaking process of nanotubes. In particular, I studied the effect of defects, stretching velocity, etc. My simulations give a clear justification for the properties observed in the experiment.