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Manuscript Title: SaX: An open source package for electronic-structure and optical-properties calculations in the GW approximation
Authors: Layla Martin-Samos, Giovanni Bussi
Program title: SaX (Self-energies and eXcitations)
Catalogue identifier: AEDF_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 180(2009)1416
Programming language: FORTRAN, plus some C utilities.
Computer: Linux PC, Linux clusters, IBM-SP5.
Operating system: Linux, Aix.
Has the code been vectorised or parallelized?: Yes
RAM: depending on the system complexity
Keywords: electronic properties, optical properties, GW self-energy, Bethe-Salpeter, plane-waves, object-oriented programming.
PACS: 71.15.-m, 71.15.Qe, 71.35.-y.
Classification: 7.3.

External 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.

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 Nv and Nc are the number of valence and conduction bands implied in the transition and Nk 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 distriburion 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 1600Mb per processor.