Libxc is a library of exchange-correlation functionals for density-functional theory. The aim is to provide a portable, well tested and reliable set of exchange and correlation functionals that can be used by all the ETSF codes and also other codes.

In Libxc you can find different types of functionals: LDA, GGA, hybrids, and mGGA. These functionals depend on local information, in the sense that the value of the potential at a given point depends only on the values of the density – and the gradient of the density and the kinetic energy density, for the GGA and mGGA cases – at a given point:

$$ E^\mathrm{LDA}_\mathrm{xc} = E^\mathrm{LDA}_\mathrm{xc}[n(\vec{r})]\,, $$

$$ E^\mathrm{GGA}_\mathrm{xc} = E^\mathrm{GGA}_{xc}[n(\vec{r}), \vec{\nabla}n(\vec{r})]\,, $$

$$ E^\mathrm{Hyb}_\mathrm{xc} = a_x E^\mathrm{EXX} + E^\mathrm{GGA}_\mathrm{xc}[n(\vec{r}), \vec{\nabla}n(\vec{r})]\,, $$

$$ E^\mathrm{mGGA}_\mathrm{xc} = E^\mathrm{mGGA}_\mathrm{xc}[n(\vec{r}), \vec{\nabla}n(\vec{r}), \nabla^2 n(\vec{r}), \tau(\vec{r})]\,. $$

It can calculate the functional itself and its derivative; for most functionals, higher-order derivatives are available.

Libxc is written in C and has Fortran and Python bindings. It is released under the MPL license (v. 2.0). Contributions are welcome.

At the moment, Libxc is used in the following codes (in alphabetical order):

- Abinit - plane-wave code
- ADF - Slater-type orbitals code
- APE - an atomic code
- Atomistix ToolKit - numerical orbitals code
- AtomPAW - projector augmented wave functions generator
- BAGEL - quantum chemistry program
- BigDFT - wavelet code
- CP2K - A program to perform atomistic and molecular simulations of solid state, liquid, molecular, and biological systems.
- DP - Dielectric Properties, a linear response TDDFT code
- Elk - FP-LAPW code
- ERKALE - a DFT/HF molecular electronic structure code based on Gaussian orbitals
- exciting - FP-LAPW code
- FHI-AIMS - full-potential, all-electron numeric atom-centered orbital code (available in next release)
- GPAW - grid-based projector-augmented wave method
- Horton - Python development platform for electronic structure methods
- JDFTx - plane-wave code designed for Joint Density Functional Theory
- MADNESS - a multiwave adaptive numerical grid program
- MOLGW - a small, but accurate MBPT code for molecules
- MRCC - a suite of ab initio and density functional quantum chemistry programs for high-accuracy electronic structure calculations
- Octopus - real-space (TD)DFT code
- PROFESS - orbital-free density functional theory (OFDFT) implementation to simulate condensed matter and molecules
- Psi4 - suite of ab initio quantum chemistry programs
- PySCF – ab initio computational chemistry program
- Quantum Espresso - a density-functional code based on plane waves and pseudopotentials
- WIEN2k - FP-LAPW code
- Yambo - solid state and molecular physics many-body calculations code

To cite Libxc, you can use the following reference

- Susi Lehtola, Conrad Steigemann, Micael J.T. Oliveira, and Miguel A.L. Marques,
*Recent developments in Libxc - A comprehensive library of functionals for density functional theory*, Software X**7**, 1 (2018) - Miguel A. L. Marques, Micael J. T. Oliveira, and Tobias Burnus,
*Libxc: a library of exchange and correlation functionals for density functional theory*, Comput. Phys. Commun.**183**, 2272 (2012).

Questions about the library can be sent to the Libxc mailing list. Bug reports and patches should be submitted over gitlab.