Potential Functions of Al2 by the Relativistic Fock-Space Coupled Cluster Method

Abstract: Potential functions of the ground and low excited states of Al₂ are calculated by the relativistic Fock-space coupled cluster method in the framework of the projected Dirac-Coulomb Hamiltonian. A moderate-size basis [16s11p3d3f/6s6p3d2f] is used. ³Π_u is confirmed as the ground state of the system. Its spin orbit splittings are reproduced well, with the Λ = 1, 2 states lying 32.5 and 66.1 cm⁻¹, respectively, above the Λ = 0 minimum (experimental values are 30.4 and 63.4 cm⁻¹). The bond is somewhat too weak, with D_e 0.14 eV below experiment, R_e too high by 0.08 ˚A, and ω_e 21 cm⁻¹ too low. It is speculated that the better agreement obtained in earlier calculations may be due to neglect of basis set superposition errors. The description of bonding in the molecule may be improved by the use of a better basis and the inclusion of more correlation by the intermediate Hamiltonian coupled cluster method, which makes it possible to handle larger P spaces and extend the potential functions to the whole range of internuclear separations.