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Manuscript Title: The VOLSCAT package for electron and positron scattering of molecular targets: a new high throughput approach to cross-section and resonances computation
Authors: N. Sanna, I. Baccarelli, G. Morelli
Program title: VOLSCAT V1.0
Catalogue identifier: AEEW_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 180(2009)2550
Programming language: Fortran90.
Computer: All SMP platforms based on AIX, Linux and SUNOS operating systems over SPARC, POWER, Intel Itanium2, X86, em64t and Opteron processors.
Operating system: SUNOS, IBM AIX, Linux RedHat (Enterprise), Linux SuSE (SLES).
Has the code been vectorised or parallelized?: Yes. The parallel version in the present release of the code is limited to the OpenMP calculation of the exchange potential Vfege or Vsce. The number of OpenMP threads can then be set in the input script.
RAM: For a typical (isolated) biomolecule (e.g. Cytosine or Ribose) a converged calculation would require from 320 MB up to 2.5 GB
Word size: 64 bits
Keywords: Single Center Expansion Library, SCE molecular properties, electron-molecule scattering.
PACS: 34.50Gb, 34.80Bm.
Classification: 16.5.

External routines: LAPACK (dgetri,dgetrf) (http://www.netlib.org/lapack/)

Nature of problem:
In this set of codes an efficient procedure is implemented to calculate partial cross section for the scattering between an electron/positron and a molecular target as a function of the collision energies.

Solution method:
The scattering equations are derived in the framework of the Single Center Expansion (SCE) procedure which allows the reduction of the original three-dimensional problem to a radial (one-dimensional) equation through the expansion of the scattering potential and the system wavefunction in a set of symmetry-adapted (real) spherical harmonics. The local part of the electrostatic interaction between the charged projectile (electron/positron) and the molecular target is provided in input by the SCELib library, which also provides the correlation and polarisation corrections for the short-range and long-range part, respectively, of the interaction. A proper Application Programming Interface (API) is used by VOLSCAT to load the energy-independent part of the potential while the non-local exchange contribution is approximated by a local form and calculated on the fly in the VOLSCAT run for each desired collision energy. The resulting SCE one-dimensional homogeneous scattering equation is rewritten in an integral form by means of the standard Green's function technique resulting in a set of Volterra coupled equations which are solved to give the phase shifts and cross sections for any desired impact energy in terms of the partial components defined by the irreducible representations of the symmetry point group to which the target molecule belongs. The total cross section can then be straightforwardly calculated by summing over all the partial cross sections produced in the output. By the Breit-Wigner analysis of the eigenphase sum produced as a function of the energy one can also get information on the location of possible resonance states arising in the collision process.

Restrictions:
Depending on the molecular system under study and on the operating conditions the program may or may not fit into available RAM memory.

Additional comments:
A beta version of SCELib-API is included in the distribution package.

Running time:
The execution time strongly depends on the molecular target description and on the hardware/OS chosen, it is directly proportional to the (τ, θ, φ) grid size and to the number of angular basis functions used.