octopus employs pseudopotentials and real-space numerical grids to propagate the Kohn–Sham orbitals in real time under the influence of time-varying electromagnetic fields. Specific functionality is provided for simulating one-, two-, and three-dimensional systems. octopus can calculate static and dynamic polarizabilities and first hyperpolarizabilities, static magnetic susceptibilities, absorption spectra, and perform molecular dynamics simulations with Ehrenfest and Car–Parrinello methods.
- Linear optical (i.e. electronic) response of molecules or clusters, also second-order nonlinear response.
- Non-linear response to classical high-intensity electromagnetic fields, taking into account both the ionic and electronic degrees of freedom.
- Ground-state and excited state electronic properties of systems with lower dimensionality, such as quantum dots.
- Photo-induced reactions of molecules (e.g., photo-dissociation, photo-isomerization, etc.).
- In the immediate future, extension of these procedures to systems that are infinite and periodic in one or more dimensions (polymers, slabs, nanotubes, solids), and to electronic transport.
- 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.
- As numerical representation, the code works without a basis set, relying on numerical meshes. Nevertheless, auxiliary basis sets (plane waves, atomic orbitals) are used when necessary. Recently, the code offers the possibility of working with non-uniform grids, which adapt to the inhomogeneity of the problem, and of making use of multigrid techniques to accelerate the calculations.
- For most calculations, the code relies on the use of pseudopotentials of two types: Troullier-Martins, and Hartwigsen-Goedecker-Hutter.
- In addition to being able to treat systems in the standard 3 dimensions, 2D and 1D modes are also available. These are useful for studying, e.g., the two-dimensional electron gas that characterizes a wide class of quantum dots.
- The code has been designed with emphasis on parallel scalability. In consequence, it allows for multiple task divisions.
- The language of most of the code is Fortran 90 (almost 50.000 lines at present). Other languages, such as C or Perl, are also used.
- The package is licensed under the GNU General Public License (GPL). In consequence, it is available for use, inspection, and modification for anyone, at the octopus web page.
- Castro, Alberto; Heiko Appel; Micael Oliveira; Carlo A. Rozzi; Xavier Andrade; Florian Lorenzen; M. A. L. Marques; E. K. U. Gross; Angel Rubio (2006). "octopus: a tool for the application of time-dependent density functional theory". physica status solidi (b). 243 (11): 2465–2488. doi:10.1002/pssb.200642067.
- W. E. Pickett, Comput. Phys. Rep. 9, 115 (1989).
- N. Troullier and J. L. Martins, Phys. Rev. B 43, 1993 (1991).
- C. Hartwigsen, S. Goedecker and J. Hutter, Phys. Rev. B 58, 3641 (1998).