SIMPLE code: Optical properties with optimal basis functions

We present SIMPLE, a code developed to calculate optical properties of metallic and insulating extended systems using the optimal basis method originally proposed by E.L. Shirley in 1996. Two different approaches for the evaluation of the complex dielectric function are implemented: (i) the independent-particle approximation considering both interband and intraband contributions for metals and (ii) the Bethe-Salpeter equation for insulators. Since, notably, the optimal basis set is systematically improvable, accurate results can be obtained at a strongly reduced computational cost. Moreover, the simplicity of the method allows for a straightforward use of the code; improvement of the optimal basis and thus the overall precision of the simulations is simply controlled by one (for metals) or two (for insulators) input thresholds. The code is extensively tested, in terms of verification and performance, on bulk silver for metals and on bulk silicon for insulators. Program summary Program Title: SIMPLE P rogram Files doi: http://dx.doi.org/10.17632/99ps9dz428.1 Licensing provisions: GNU General Public License v2 Programming language: Fortran 95 External routines: Quantum ESPRESSO distribution, BLAS, LAPACK, FFTW, MPI. Nature of problem: First-principles calculations of the optical properties of metals and insulators. Solution method: Shirley's optimal basis for the complex dielectric function, at the independent-particle level for metals and by solving the Bethe Salpeter equation for insulators. Restrictions: Norm-conserving pseudopotentials. (C) 2019 Elsevier B.V. All rights