Energy and density analysis on the H2 molecule from the united atom to dissociation: The Σ, Π, Δ, ϕ, and Γ manifolds

Abstract

We present 140 accurate potential energy curves, PECs, for the Σ, Π, Δ, ϕ, and Γ manifolds for the H2 molecule, mapping all the states with energy below the H ground state. The full configuration interaction, nonrelativistic Born–Oppenheimer computations are performed with large and optimized basis sets of Slater-type and spherical Gaussian functions; these new basis sets are somewhat larger than those used in recent published studies on the 60 Σ state PECs. The full CI computations are performed twice, with Hartree–Fock and with Heitler–London-type functions, allowing the identification of the ionic component in the total energy. The computed energies are within 10−5 hartree from the most accurate PECs in literature. We aim (a) at the evaluation of the PECs starting at very short and unexplored internuclear distances (0.01 bohrs) and ending at full dissociation, (b) at the systematic prediction of high excited state PECs dissociating as 1s + 4l and 1s + 5l, and (c) at the characterization of the evolution of the 140 PEC electronic densities from united atom to dissociation. With this work we fill a gap in today literature, which has dealt mainly with low excited states, generally excluding short internuclear distances. The electronic configuration at the united atom persists as dominant configuration well beyond the equilibrium separation, and it switches to that at dissociation often with energy patterns seemingly irregular, in particular when the values of the principal quantum number at dissociation and at the united atom differ by one or more unit. The Hund's singlet-triplet splitting, which propagates from the united atom to the molecule, is discussed. The singlet and triplet states are rather close in energy in the Π manifolds, and approach degeneracy in the Δ and ϕ manifolds, to become fully degenerate in the Γ manifolds. Discussions on the correlation energy correction, adiabatic correction, spectroscopic constants and on general features of the H2 excited states are presented. The H2 molecule is a system, which—to be understood—needs consideration of both the very short internuclear distances in approaching the united atom and of the very high excited states below H. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011