RT-SIESTA: A real-time time-dependent density functional theory code for the computation of linear and non-linear optical responseIn recent years there have been major advances in the design of novel organic materials for solar energy utilization and telecommunications/information systems. These advances have highlighted the need for new theoretical methods to facilitate the rational design of these materials. In particular, they have emphasized the need for modern scientific software to compute the response of arbitrary materials to electromagnetic radiation. Developed by the Rehr group in the University of Washington, RT-SIESTA endeavors to address these challenges in an efficient way.
RT-SIESTA implements the time-dependent density functional theory (TDDFT) by propagating the DFT wavefunctions in real-time, and computing the linear and non-linear response to a wide variety of external perturbations. The propagation uses a predictor-corrector Crank-Nicolson (PCCN) algorithm which ensures the time-reversibility of the approach, resulting in a very stable implementation allowing for long time steps. A variety of external electric field perturbations are available depending on the target response. For instance, impulse and step perturbations excite the UV-Vis region, resulting in complete spectra from single calculations. More sophisticated perturbations such as enveloped sine waves provide non-linear optical properties at specific frequencies. Typical properties include the frequency-dependent linear dipole-polarizability and non-linear hyperpolarizability, and such optical processes as Second Harmonic Generation (SHG), Hyper-Rayleigh Scattering (HRS) and Optical Rectification (OR). Current development is focused on implementing nuclear potential perturbations aimed at studying core-excitation response (See, for instance, the RTXS code.) The code is an extension of the popular SIESTA code, and takes advantage of its efficient linear combination of atomic orbitals (LCAO) basis set, and use of pseudopotentials. Together with the PCCN approach, these allows for routine simulation of systems with up to 300 atoms. Given that solvent effects are an integral part of most experimental measurements of optical response, we have also improved the standard SIESTA distribution by implementing the Onsager continuum solvation model.