octopus (software)

• 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{{cite journal | last=Pickett | first=Warren E. | title=Pseudopotential methods in condensed matter applications | journal=Computer Physics Reports | publisher=Elsevier BV | volume=9 | issue=3 | year=1989 | issn=0167-7977 | doi=10.1016/0167-7977(89)90002-6 | pages=115–197| bibcode=1989CoPhR...9..115P }} of two types: Troullier-Martins,{{cite journal | last1=Troullier | first1=N. | last2=Martins | first2=José Luriaas | title=Efficient pseudopotentials for plane-wave calculations | journal=Physical Review B | publisher=American Physical Society (APS) | volume=43 | issue=3 | date=1991-01-15 | issn=0163-1829 | doi=10.1103/physrevb.43.1993 | pages=1993–2006| pmid=9997467 | bibcode=1991PhRvB..43.1993T }} and Hartwigsen-Goedecker-Hutter.{{cite journal | last1=Hartwigsen | first1=C. | last2=Goedecker | first2=S. | last3=Hutter | first3=J. | title=Relativistic separable dual-space Gaussian pseudopotentials from H to Rn | journal=Physical Review B | publisher=American Physical Society (APS) | volume=58 | issue=7 | date=1998-08-15 | issn=0163-1829 | doi=10.1103/physrevb.58.3641 | pages=3641–3662| arxiv=cond-mat/9803286 | bibcode=1998PhRvB..58.3641H | s2cid=119450371 }}
• 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, this utilises mesh division software, MPI and OpenMP.
• The language of most of the code is Fortran 90. Other languages, such as C, 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 [https://gitlab.com/octopus-code/octopus Octopus git repository].

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• Quantum chemistry computer programs

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Category:Free software programmed in Fortran

Category:Computational chemistry software

Category:Free science software

Category:Software using the GNU General Public License