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Manuscript Title: Global fit of ab initio potential energy surfaces: II.1. Tetraatomic systems ABCD.
Authors: A. Aguado, C. Tablero, M. Paniagua
Program title: GFIT4C
Catalogue identifier: ADNB_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 134(2001)97
Programming language: Fortran.
Computer: IBM RISC 6000.
Operating system: AIX
RAM: 4920K words
Word size: 32
Keywords: Global potential energy surface (GPES), Tetraatomic 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 global 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 the part I of this series we have dealt with triatomic systems and three-dimensional (3D) representations. In the present part II.1 of this series we treat the ABCD class of tetraatomic systems and six-dimensional (6D) representations. The GFIT4C program contains drive code to control the the five cases for tetraatomic systems. However, in this program, we have developed the subroutines corresponding to ABCD class. The following parts II (ABC2, A2B2, AB3 and A4 class of tetraatomic systems are in preparation), and III (program GFIT5C, for pentaatomic (9D) systems is 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- 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 some of the authors [2]. Then, the GFIT4C and specific subroutines of the GFIT3C programs fit the four-body term to the ab initio values of the tetraatomic system ABCD minus the diatomics and triatomics 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 different classes of tetraatomic and pentaatomic systems).

The program GFIT4C is applicable to general tetraatomic systems, i.e. ABCD class, without nuclei permutational symmetry, but it is also applicable to tetraatomic systems with several identical nuclei with a worse behaviour and efficiency than if the permutational symmetry were taken into account. The subroutines for different classes of tetraatomic systems with nuclei permutational symmetry will be presented in the following parts II of this series. The program GFIT4C presented here 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- 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 32568 CPU seconds (in an IBM RISC 6000/3CT workstation) including input/output time.

[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. Tablero and M. Paniagua. Global fit of ab initio potential energy surfaces: I. Triatomic systems. Comput. Phys. Commun. 108, 259-266 (1998).