  The last ten years have witnessed fast spreading of massively parallel
computing clusters, from leading supercomputing facilities down to the average
university computing center. Many companies in the private sector have
undergone a similar evolution. In this scenario, the seamless integration of
software and middleware libraries is a key ingredient to ensure portability of
scientific codes and guarantees them an extended lifetime. In this work, we
describe the integration of the ChASE library, a modern parallel eigensolver,
into an existing legacy code for the first-principles computation of optical
properties of materials via solution of the Bethe-Salpeter equation for the
optical polarization function. Our numerical tests show that, as a result of
integrating ChASE and parallelizing the reading routine, the code experiences a
remarkable speedup and greatly improved scaling behavior on both multi- and
many-core architectures. We demonstrate that such a modernized BSE code will,
by fully exploiting parallel computing architectures and file systems, enable
domain scientists to accurately study complex material systems that were not
accessible before.
