CONUNDrum: A program for orbital-free density functional theory calculations

We present a new code for energy minimization, structure relaxation and evaluation of bulk parameters in the framework of orbital-free density functional theory (OF-DFT). The implementation is based on solving the Euler–Lagrange equation on an equidistant real space grid on which density dependent variables and derivatives are computed. Some potential components are computed in Fourier space. The code is able to use semilocal and non-local kinetic energy functionals (KEF) as well as neural network based KEFs thus facilitating testing and development of emerging machine-learned KEFs. For semi-local and machine-learned KEFs the kinetic energy potentials are evaluated with real-space differentiation of the components, which are partial derivatives of the KE with respect to the electron density, its gradient and Laplacian.