# DftFunctionals

DftFunctionals provides an Interface for evaluating exchange-correlation functionals for density-functional theory calculations.

For a small number of functionals a Generic functional implementation in pure Julia is also provided, where the focus for the moment is on readable code rather than speed.

The package is at an early stage, so please report bugs or numerical issues if you find them.

A more stable (but less generic) implementation of DFT functionals is available via the Libxc.jl package. Currently Libxc is not yet compatible with DftFunctionals out of the box (but there exists an experimental wrapper in DFTK.jl). This will change in the future.

## Usage

Input data to DftFunctionals is the density $ρ$ (and its derivatives such as $σ = \nabla \rho \cdot \nabla \rho$ etc.) on a real-space grid. This data needs to be passed as a (n_spin, n_p) matrix, where n_spin is the number of spin components and n_p is the number of data points. Notice that three-dimensional density therefore need to be flattened before calling anything from DftFunctionals.jl. For more details on the shapes of the input and output arrays, see the documentation of potential_terms.

using DftFunctionals

# Setup dummy input data
ρ = reshape([0.1, 0.2, 0.3, 0.4, 0.5], 1, :)
σ = reshape([0.2, 0.3, 0.4, 0.5, 0.6], 1, :)

# Setup the functional to use
xc = DftFunctional(:gga_x_pbe)

# Get the potential terms
terms = potential_terms(xc, ρ, σ)

# Energy per unit volume on each grid point
@show terms.e

# Potential Vσ = de / dρ on each grid point
@show terms.Vρ

nothing
terms.e = [-0.04525975233085996, -0.09577557406559699, -0.15620226503032583, -0.22457111882837902, -0.2993306248309841]
terms.Vρ = [-0.42731424469320173 -0.5301998810629429 -0.6288348093786362 -0.7043765616395834 -0.7658496683995673]