Octopus (software) - Theoretical Basis

Theoretical Basis

• The underlying theories are DFT and TDDFT. Also, the code may perform dynamics by considering the classical (i.e. point-particle) approximation for the nuclei. These dynamics may be non-adiabatic, since the system evolves following the Ehrenfest path. It is, however, a mean-field approach.

• Regarding TDDFT, one can use three different approaches:
  • the standard TDDFT-based linear-response theory of Casida, which provides the excitation energies and oscillator strengths for ground-state to excited-state transitions.
  • the explicit time-propagation of the TDDFT equations, which allows for the use of large external potentials, well beyond the range of validity of perturbation theory.
  • the Sternheimer equation (density-functional perturbation theory) in the frequency domain, using only occupied states.