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.我们提出了SIMPLE程序，该程序基于E. L. Shirley于1996年提出的最优基方法（optimal basis method），用于计算金属与绝缘性延展体系的光学性质。该程序实现了两种用于计算复介电函数（complex dielectric function）的不同方案：(i) 针对金属的独立粒子近似（independent-particle approximation），兼顾带间与带内跃迁贡献；(ii) 针对绝缘体的贝特-萨尔皮特方程（Bethe–Salpeter equation）。值得注意的是，最优基组（optimal basis set）具备系统性可改进性，因此可在大幅降低计算开销的前提下获得高精度结果。此外，该方法的简洁性使得程序易于使用；通过仅调整1个（针对金属）或2个（针对绝缘体）输入阈值（input thresholds），即可实现最优基组的改进，进而提升模拟的整体精度。本程序已针对金属与绝缘体分别开展了充分的验证与性能测试：金属体系以块体银（bulk silver）为测试对象，绝缘体体系则以块体硅（bulk silicon）为测试对象。