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Manuscript Title: Global fit of ab initio potential energy surfaces: II.2. Tetratomic systems A2B2 and ABC2.
Authors: C. Tablero, A. Aguado, M. Paniagua
Program title: GFIT4C-2
Catalogue identifier: ADOV_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 140(2001)412
Programming language: Fortran.
Computer: IBM RISC 6000/3CT.
Operating system: AIX
RAM: 12K words
Word size: 32
Keywords: Global potential energy surface (GPES), Tetratomic systems, Molecular dynamical calculations, Structure.
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 golbal analytical (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 Part II.1 [4] of this series we have dealt with the tetratomic system class ABCD and six-dimensional (6D) representation (program GFIT4C). In the present Part II.2 we treat A2B2 and ABC2 tetratomic system classes and 6D representations (program GFIT4C-2) that are invariant with respect to permutations of two equivalent nuclei.
The GFIT4C program contains drive code to control the five cases for tetratomic systems. However, in the program here reported, we have developed the subroutines corresponding to ABC2, and A2B2 system classes. The following versions of the program (Parts II.3 and II.4 of this series) deal with the AB3 and A4 system classes of tetratomic molecules, giving potentials that are invariant with respect to permutations of three and four equivalent nuclei, respectively. These new versions are in preparation.

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 and three-body terms in the many-body expansion to the corresponding ab initio data for all the possible diatomics and triatomics molecules, using the functional form proposed by part of the authors [2]. Then, the GFIT4C program fits the four-body term to the ab initio values of the tetratomic system (including system classes A2B2, ABC2 and ABCD in the version GFIT4C-2 of the program) minus the diatomics and triatomics potentials evalued at the corresponding internuclear distances, using symmetry adapted product functions.

These new subroutines of the program GFIT4C are only applicable to tetratomic A2B2 and ABC2 system classes, with permutational symmetry between two identical nuclei, but it is also applicable to tetratomic systems with more than two identical nuclei (whose subroutines will be presented in Parts II.3 and II.4 of this series) with a worse behaviour and efficiency than if the whole permutational symmetry were taken into account. These new subroutines are needed also to run the following programs GFIT5C of the corresponding series for pentatomic systems. The program GFIT4C is dimensioned for a maximum of 2000 ab initio points and a maximum degree of 15 for the two-body fitting polynomials and 10 for the three- and four-body fitting polynomials. However, the dimension corresponding to the number of ab initio points may be enlarged easily by modifying the value of "NMAX" parameter in the file dimensions.inc.

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

Running time:
For the test deck is about 2065 and 2298 CPU seconds (in an IBM RISC 6000/3CT workstation) including input/output time for the A2B2 and ABC2 system classes.

[1] A. Aguado, M. Paniagua, A new functional form to obtain analytical potentials of triatomic molecules, J. Chem. Phys. 96 (1992) 1265-1275.
[2] A. Aguado, C. Suarez, M. Paniagua, Accurate global fit of the H4 potential energy surface, J. Chem. Phys. 101 (1994) 4004-4010.
[3] A. Aguado, C. Tablero, M. Paniagua, Global fit of ab initio potential energy surfaces: I. Triatomic systems, Comput. Phys. Comm. 108 (1998) 259-266.
[4] A. Aguado, C. Tablero, M. Paniagua, Global fit of ab initio potential energy surfaces: II.1. Tetraatomic systems ABCD, Comput. Phys. Comm. 134 (2001) 97-109.