Authors and Contributors

The EXC code is today being released under the GPLv2 licenced by Lucia Reining, Valerio Olevano, Francesco Sottile, Stefan Albrecht, Giovanni Onida, who are the official authors of the code (copyright (C) 1992-2011).

The present team of developers include Matteo Gatti.

The EXC code was firstly created in 1994, by Lucia Reining and Giovanni Onida. Of course this code has not been born in empty space. In particular, the first version has profited from our previous knowledge of Rex Godby's GW code. Discussions and interactions with people working in the field have been invaluable, in particular a strong collaboration with Rodolfo Del Sole. The first EXC code only dealt with Gamma point, so it was in principle possible to calculate the excitonic spectra of finite systems only. The result of this first version of the code was however important: the first ab-initio application of the Bethe-Salpeter equation to the calculation of the optical absorption spectrum of a cluster: Na₄.

The method has shown its capability to correctly reproduce excitons and excitonic effects. It was during his PhD thesis that Stefan Albrecht generalized the code in order to use k-points and symmetries. Since then, the approach has been applied to calculate the optical spectrum with excitonic effects of bulk insulators (Li₂O), semiconductors (Si) and to calculate the Electron Energy Loss spectrum of semiconductors (Si). Stefan seems to have understood better than us, because he left physics after his PhD, and has been working in an insurance company :-)

Thanks to the adoption of Fortran 90 with some C insertions, the contribution of Valerio Olevano gave the code the actual (hopefully) user-friendly form (structure of the invocation and of the input/output, structure of the compilation makefile and link to the math libraires). Also he rewrote some main parts of the code and many libraries, with the purpose to make more user-friendly the sources too. Beyond cosmetics, he implemented the calculation of the real part of the dielectric function (with the purpose to calculate all quantities that depends on the real part, like, e.g. the refraction indices or the electron energy loss spectrum (EELS)), the coupling terms, and some perturbative resolutions of the BSE.

Finally Francesco Sottile worked firstly on functionality and portability, creating a manual, this website, the actual three-stage rocket structure, with the configure script; secondly he worked on the Haydock iterative diagonalisation (with Margherita Marsili) and the extension to finite momentum tranfer (with Matteo Gatti); thirdly he parallelised the code (both following the Openmp and MPI philosophies). He is the present responsible for the code development.

There have been several people who contributed to the EXC code, in particular Margherita Marsili, who helped writing and implementing the Haydock iterative scheme, and Matteo Gatti, who is today, with Francesco Sottile, the main developer of the code; his constributions span from the excitonic spectrum for non-zero momentum transfer, to the Haydock iterative diagonalisation for non-Hermitian matrices, to the exciton analysis tools.