SaX: An open source package for electronic-structure and optical-properties calculations in the GW approximation

We present here SaX (Self-energies and eXcitations), a plane-waves package aimed at electronic-structure and optical-properties calculations in the GW framework, namely using the GW approximation for quasi-particle properties and the Bethe–Salpeter equation for the excitonic effects. The code is mostly written in FORTRAN90 in a modern style, with extensive use of data abstraction (i.e. objects). SaX employs state of the art techniques and can treat large systems. The package is released with an open source license and can be also download from http://www.sax-project.org/.Program summaryProgram title: SaX (Self-energies and eXcitations)Catalogue identifier: AEDF_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEDF_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: GNU General Public LicenseNo. of lines in distributed program, including test data, etc.: 779 771No. of bytes in distributed program, including test data, etc.: 4 894 755Distribution format: tar.gzProgramming language: FORTRAN, plus some C utilitiesComputer: Linux PC, Linux clusters, IBM-SP5Operating system: Linux, AixHas the code been vectorised or parallelized?: YesRAM: depending on the system complexityClassification: 7.3External routines: Message-Passing Interface (MPI) to perform parallel computations. ESPRESSO (http://www.quantum-espresso.org)Nature of problem: SaX is designed to calculate the electronic band-structure of semiconductors, including quasi-particle effects and optical properties including excitonic effects.Solution method: The electronic band-structure is calculated using the GW approximation for the self-energy operator. The optical properties are calculated solving the Bethe–Salpeter equation in the GW approximation. The wavefunctions are expanded on a plane-waves basis set, using norm-conserving pseudopotentials.Restrictions:   Many objects are non-local matrix represented in plane wave basis sets. The memory required by the program in the allocation of such objects increases with the increase of the simulation cell volume. Other quantities are built calculating electronic transitions, so that the computational time increase with their number, and scales as Nv×Nc×Nk2, where NvNv and NcNc are the number of valence and conduction bands implied in the transition and NkNk is the number of special k vectors. Symmetries are not exploited yet. Finally, metallic systems cannot be studied yet.Unusual features: SaX is written using FORTRAN90 in an object-oriented way. Thus, it is easy to add new features and to reuse the code.Running time: The 3 examples, contained in the distribution file, each take only a few seconds to run. For systems of interest, the run may take a number of days with a typical memory allocation of 1600 Mb per processor.