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Manuscript Title: Global fit of ab initio potential energy surfaces: I. Triatomic systems.
Authors: A. Aguado, C. Tablero, M. Paniagua
Program title: GFIT3C
Catalogue identifier: ADHK_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 108(1998)259
Programming language: Fortran.
Computer: IBM RISC 6000/3CT.
Operating system: AIX
RAM: 653K words
Word size: 32
Keywords: Potential energy surface (PES), Global fit, Molecular dynamical calculations.
Classification: 16.1.

Nature of problem:
Given a set of ab initio points of a molecular system with N atoms, the problem is to obtain a gloabal analytic (3N - 6)-dimensional representation of the corresponding adiabatic potential having all the symmetry properties of the system and satisfying the stringent criteria [1] needed in molecular dynamical calculations. In particular, in the part I of this series we deal with triatomic systems and three- dimensional (3D) representations. Parts II and III of this series of papers will be concerned with global PES for tetraatomic (6D) and pentaatomic (9D) systems, respectively. Part II (program GFIT4C, in preparation) requires part I as a prerequisite, part III (program GFIT5C, in project) should need subroutines from GFIT3C and GFIT4C.

Solution method:
The method of solution consists in expressing the potential as a many- body expansion choosing as variables the internuclear distances. The program sequentially fits all the two-body terms in the many-body expansion to the corresponding ab initio data for all the possible diatomic molecules, using the functional form proposed by part of the authors [2]. Then, the GFIT3C program fits the three-body term to the ab initio values of the triatomic system minus the diatomic potentials evaluated at the corresponding internuclear distances, using symmetry adapted product functions as for tetraatomic systems [3]. Similar methods will be used in the following programs of this series (parts II and III for tetraatomic and pentaatomic systems).

The program GFIT3C is only applicable to triatomic systems, but their subroutines are needed to run GFIT4C and GFIT5C (parts II and III of this series), for tetraatomic and pentaatomic systems, respectively. The program GFIT3C is dimensioned for a maximum of 2,000 ab initio points and a maximum degree of 15 for the two-body fitting polynomials and 10 for the three-body fitting polynomials.

Unusual features:
Fortran-77 IBM INCLUDE compiler directive is used.

Running time:
For the test deck is about 1 CPU second (in an IBM RISC 6000/3CT workstation) including input/output time. However, this is for only one nonlinear parameter.

[1] A. Aguado and M. Paniagua. A new functional form to obtain analytical potentials of triatomic molecules. J. Chem. Phys., 96, 1265-1275 (1992).
[2] A. Aguado, C. Suarez and M. Paniagua. Accurate global fit of the H4 potential energy surface. J. Chem. Phys., 101, 4004-4010 (1994).
[3] A. Aguado, C. Suarez, M. Paniagua, Accurate global fit of the H4 potential energy surface, J. Chem. Phys. 101 (1994) 4004-4010.