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RTTDDFT

This page gives some information on how to perform a real-time time-dependent DFT (RT-TDDFT) calculation with the ABINIT package.

Warning

RT-TDDFT is under active development and should thus be used with caution!

Introduction

The goal of TDDFT is usually to describe the electronic response to an external time-dependent perturbation. To do so, real-time TDDFT numerically integrates the time-dependent Kohn-Sham (TDKS) equations in real-time and thus gives access to the time evolution of the electronic density directly. Similarly to the widely used linear-response TDDFT approach (see topic_TDDFT), this method can be used to compute the electronic response in the linear regime giving access among others to transport coefficients, such as the electrical conductivity, and optical properties, such as the absorption spectrum. Moreover, RT-TDDFT is not restricted to the linear regime, and can thus be used to study the response to intense perturbations. This method is thus particularly suited to investigate the non-equilibrium electron dynamics following an intense excitation such as the response to high intensity lasers. It has been successfully used to study different phenomena including high-harmonics generation, electron stopping power, core electron excitations etc. A detailed description of TDDFT including real-time propagation schemes can be found for instance in the book of C. Ullrich [Ullrich2011].

Implementation in ABINIT

ABINIT implements RT-TDDFT in the so-called adiabatic approximation using the standard XC functionals developed for ground state calculations. The implementation works with LDA and GGA functionals. It has not yet been tested for other types (meta-GGAs, hybrids) and is thus most probably not compatible for now. Moreover, the implementation has not been yet tested on all possible cases such as magnetic cases or with spin-orbit coupling and thus should not be used in theses cases for now. The integration of the TDKS equations works with both norm-conserving pseudopotentials and the projector augmented wave (PAW) method topic_PseudosPAW.

As of now only the simple Exponential Rule (ER) and the Exponential Mid-point Rule (EMR) propagators are implemented which seem to be sufficiently stable in most cases (see for instance [Castro2004] for more information on propagators for RT-TDDFT). It is possible to apply an external impulse electric field in order to compute the associated response functions, so typically the conductivity and the dielectric function. The Tutorial on real-time TDDFT describes how to run such calculations to compute the dielectric function of Diamond. Note that the application of an external electric field is only possible in PAW for now.

compulsory:

basic:

  • dtele Delta Time for ELEctrons
  • ntime Number of TIME steps

useful:

expert:

Selected Input Files

rttddft_suite:

Tutorials