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. Program summary Program title: CONUNDrum. CPC Library link to program files: http://dx.doi.org/10.17632/phnz2gg8mz.1 Licensing provision: GNU GPL v3 Programming language: C++ External routines: Fastest Fourier Transform in the West (FFTW) library (http://www.fftw.org/) Nature of problem: Calculation of the electronic and structural properties of molecules and extended systems in the framework of the orbital-free density functional theory. Evaluation of the bulk parameters of solid compounds. Solution method: High-order central finite-difference method and fast Fourier transform are used for calculation of different total energy components. Density optimization is performed with the steepest descent or the Polak-Ribiere variant of the non-linear conjugate-gradient method with a line search procedure based on the Armijo condition. A numerical approach is used for structural optimization the total energies with respect to small variations in lattice geometries are computed directly, with subsequent evaluation of the force components via a high-order central-finite difference method. The same numerical procedure is used for evaluation of bulk properties. Restrictions: Local pseudopotentials. (C) 2020 Elsevier B.V. All rights reserved.