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Manuscript Title: SCELib: a parallel computational library of molecular properties in the single centre approach.
Authors: N. Sanna, F.A. Gianturco
Program title: SCELib
Catalogue identifier: ADMG_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 128(2000)139
Programming language: C.
Computer: Compaq ES40, SUN ES4500, IBM SP.
Operating system: Compaq True64 V4.0X, SUN Solaris V2.X, AIX V4.3.X, Linux RedHat V6.0.
RAM: 200M words
Word size: 64
Keywords: Single Centre Expansion Library, SCE molecular properties, electron molecule scattering, Molecular Physics.
Classification: 16.5.

Nature of problem:
In this set of codes an efficient procedure is implemented to describe the wavefunction and related molecular properties of a polyatomic molecular system within the Single Centre of Expansion (SCE) framework of reference. The resulting SCE wavefunction, electron density, electrostatic and exchange/correlation potentials can then be used via a proper Application Program Interface (AFI) to describe the target molecular system in electron-molecule scattering calculations. The generated molecular properties which are expanded over a single centre turn out to be of more general application and some possible uses in quantum chemistry, biomodelling and drug design are also outlined.

Solution method:
The polycentre Hartree-Fock solution for a molecule of arbitrary geometry, based on linear combination of Gaussian-Type Orbital (GT0), is expanded over a single centre, typically the Centre of Mass (C.O.M.), by means of a Gauss Legendre/Chebyschev quadrature over the theta, phi angular coordinates. The resulting SCE numerical wavefunction is then used to calculate the one-particle electron density, the electrostatic potential, a model exchange potential and two correlation/polarization potentials induced by the impinging electrons, with the correct aymptotic behaviour for the leading dipole molecular polarizabilities.

Depending on the molecular system under study and on the operating conditions the program may or may not fit into available RAM memory. In this case a feature of the program is to memory map a disk file in order to efficiently access the memory data through a disk device.

Unusual features:
The code has been engineered to use dynamical, runtime determined, global parameters with the aim to have all the data fitted in RAM memory. Some unusual circumstances, e.g. when using large values of those parameters, may cause the program to run with unexpected performance reductions due to runtime bottlenecks like memory swap operations which strongly depend on the hardware used. In these cases a parallel execution of the code is generally sufficient to fix the problem since the data size is partitioned over the available processors. When a suitable parallel system is not available for execution, a mechanism of memory mapped file can be used; with this option on, all the available memory will be used as buffer for a disk file which contains the whole data set, thus having a better throughput with respect to the traditional swapping/paging of the Unix OS.

Running time:
The execution time strongly depends on the molecular target description and on the hardware/OS chosen, but it is directly proportional to the (r,theta,phi) grid size and to the number of angular basis functions used. Thus, from the program printout of the main arrays memory occupancy, the user can approximately derive the expected computer time needed for a given calculation executed in serial mode. For parallel executions the overall efficiency must be further taken into account, and this depends on the number of processors used as well as on the parallel architecture chosen, so a simple general law is at present not determinable.