  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.
