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Manuscript Title: A fitting program for potential energy surfaces of bent triatomic molecules.
Authors: D.J. Searles, E.I. von Nagy-Felsobuki
Program title: Fit-PowPad
Catalogue identifier: ACBU_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 67(1992)527
Programming language: Fortran.
Computer: VAX 8550.
Operating system: VMS.
RAM: 1096K words
Word size: 32
Keywords: Molecular physics, Structure, General purpose, Least-squares fitting, Regression, Singular value Decomposition, Molecular properties, Power series expansions, Pade approximants, Morse expansion, Durham expansion, Ogilvie expansion, Simon-parr-finlan Expansion.
Classification: 16.1.

Nature of problem:
Electronic ab initio calculations can yield discrete potential energy surfaces for triatomic molecules. In order to solve the nuclear Schrodinger equation and so obtain information on the molecular dynamics of these molecules it is necessary to develop an analytical represent- ation of the discrete surface. We therefore present a Fortran program for least squares fitting of discrete electronic points to power series expansions (Dunham, Simon-Parr-Finlan, Ogilvie and expotential variants) as well as to Pade approximants.

Solution method:
We employ standard least squares fitting techniques using the singular value decomposition method in order to dampen the high-order coefficients (if deemed necessary) without degrading the fit significantly.

Restrictions:
The program makes full use of symmetry of a triatomic molecule and so addresses the D3h, C2v and CS cases. The program does not address the linear case.