# SIMPLE code: optical properties with optimal basis functions

## TL;DR

SIMPLE is a computational code that efficiently calculates optical properties of materials using an optimal basis method, reducing computational costs while maintaining accuracy for metals and insulators.

## Contribution

The paper introduces SIMPLE, a new code implementing the optimal basis method for optical property calculations, with systematic improvability and ease of use.

## Key findings

- Accurate optical properties obtained with reduced computational cost.
- Effective for both metallic and insulating systems.
- Validated on bulk silver and silicon.

## Abstract

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.