Ab initio study of the Li(2S)-H2(X1Σg+)van der Waals complex

The adiabatic potential energy surface (PES) of the Li(S-2)-H-2(X(1)Sigma (+)(g)) van der Waals complex, described by Jacobi coordinates (r = 0.741 Angstrom, R, Theta), was investigated using the supermolecular unrestricted fourth-order Moller-Plesset perturbation theory. Our calculations predict two equivalent minima for the linear arrangements (Theta = 0 degrees and 180 degrees). These minima were found at R = 5.22 Angstrom with a well depth of D-e = 68.55 muE(h). This implies that Li-H-2 complex is bounded very weakly. The presented PES reveals that these two minima are separated by a barrier of 25.74 muE(h) at Theta = 90 degrees and R = 5.42 Angstrom. The physical origin of the studied weak interaction was analysed by the intermolecular perturbation theory based on the single determinant UHF wave function. The separation of the interaction energy shows that the locations of the predicted stable structures are primarily determined by anisotropy of the repulsive Heitler-London exchange penetration and attractive dispersion + induction energy components. (C) 2001 Elsevier Science B.V. All rights reserved.