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Manuscript Title: VisRes: a GRACE tool for displaying and analysing resonances.
Authors: D.W. Busby, P.G. Burke, V.M. Burke, C.J. Noble, N.S. Scott
Program title: VisRes 2.00
Catalogue identifier: ADJE_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 114(1998)243
Programming language: C, Fortran, Motif.
Computer: SUN.
Operating system: Solaris 2.5, IRIX5.3, HP-UX9.05, ULTRIX4.3, AIX2.3.
RAM: 16M words
Keywords: Atomic physics, Scattering, Electron, General purpose, Fit, Farm, Grace, Osf/motif, Resonance, R-matrix.
Classification: 2.4, 4.9.

Nature of problem:
Electron and photon collisions with atoms and molecules give rise to quasi-bound states with long lifetimes. These resonances are exhibited in theoretical computations by a rapid increase in the eigenphase sum of approximately pi radians. VisRes is a graphical tool designed to enable the user to read files of eigenphase sums, to detect resonances, and to determine accurately their positions and widths.

Solution method:
Detecting and analysing resonances from a file of energy dependent eigenphase sums is difficult for two reasons. First, there are often insufficient data points in the vicinity of a resonance to determine its characteristics accurately. Second, rapid changes in an eigenphase sum can result from the transition from one arctan branch to another, rather than from a resonance. VisRes is a graphical tool designed to circumvent these difficulties. It enables files of eigenphase sums to be read and the data displayed as discontinuous graphs, over a range of pi radians, and as continuous graphs, over a range of npi radians. Additional points needed to resolve a resonance can be computed interactively by invoking FARM [1] or a similar program and merging the new data into the displayed graphs. Resonance positions and widths are determined by fitting interactively selected data points by the Breit- Wigner formula [2]. The fitting is performed by the Fortran subroutine SNLS1E from the public domain SLATEC Library [3]. It minimizes the sum of the squares of M nonlinear functions in N variables by a modification of the Levenberg-Marquardt algorithm. The SNLS1E source code is included in the VisRes code distribution.

Restrictions:
Current array dimension settings impose the following restrictions: VisRes can deal with up to three overlaping resonances and a background polynomial phaseshift up to degree four; the maximum number of data points that can be used in the fitting process is restricted to 250; and the number of data points that can be computed interactively at one time is restricted to 25. Larger cases can be considered by altering the array dimensions.

Unusual features:
VisRes can be operated exclusively on the host workstation or in distributed mode where data is read from and computed on a remote computer but displayed and analysed on the host workstation.

References:
[1] V.M. Burke and C.J. Noble, Comput. Phys. Commun. 85 (1995) 471.
[2] G. Breit and E.P. Wigner, Phys. Rev. 49 (1936) 519.
[3] http://www.netlib.org/slatec.