F. D. Colavecchia, J. P. Burke, Jr., W. J. Stevens, M. R. Salazar, G. A. Parker and R. T Pack, The Potential Energy Surface for Spin-Aligned Li₃ (1 ⁴A') and the Potential Energy Curve for Spin-Aligned Li₂ (a ³S_u). J. Chem. Phys 118 5484-5495 (2003).

(69) J. Chem. Phys 118 5484-5495 (2003).pdf

Abstract: A global potential energy surface (PES) for the 1 ⁴A' spin-aligned state of Li₃ is presented. The surface is constructed as a many body expansion of the potential which is the sum of pairwise additive two-body potentials plus a three-body term. The two-body potential is that for the a ³S_u state of the lithium dimer. It combines the most recent Rydberg-Klein-Rees (RKR) potential available [A. Ross, Private Communication] with well-known short and long range expansions and accurately reproduces all known experimental data. To obtain the three-body contributions, an ab initio PES was computed at 1122 points using Full Configuration Interaction (FCI) for the three valence electrons with an augmented Gaussian basis and the Effective Core Potentials (ECP) of Stevens, Basch, and Krauss  [W. J. Stevens et al. 81, 6026 (1984)] for the other electrons. The two-body interactions are also calculated using the same basis and then subtracted from the full interaction to give the three-body term. To construct the three-body potential at arbitrary configurations we use interpolation for small perimeters of the triangle formed by the triatomic system and an analytic fitting function for large perimeters. A switching function guarantees the smoothness of the potential function everywhere. The equilibrium position occurs at D_3h symmetry with a bond distance of 5.861a₀, nearly 2a₀ smaller than the equilibrium value of 7.886 a₀ of the lithium dimer. The well depth at the equilibrium is 4112.64 cm^-1. This is considerably deeper than the well depth of 1001.22 cm^-1 for the pairwise additive potential at its equilibrium. Three-body effects are even more important for Li₃ than in the recently reported  Na₃ case [J. Higgins et al. J. Chem. Phys. 112, 5751 (2000)], and the non-additive three-body term cannot be neglected in any calculation on this system.

Zip file containing the FORTRAN code and ab initio data:  li3potential.tar.gz

Professor Gregory A. Parker
440 West Brooks
Department of Physics and Astronomy
University of Oklahoma
Norman, OK 73019 B.S. 1973

Brigham Young University
Ph.D. 1976 Brigham Young University