Ab initio pair potentials for the ionic lithium-formate system

The potential energy surfaces for the interatomic interaction in the Li⁺HCOO^- system have been investigated byab initio methods within the rigid-molecule approximation. Analytical potential expressions were fitted to 133 calculated SCF energies for the Li⁺-HCOO^- interaction, 42 SCF energies for the Li⁺-Li⁺ interaction, and 332 SCF energies for the HCOO^--HCOO^- interaction. The global minimum on the Li⁺-HCOO^- surface is -170 kcal/mol and corresponds to the lithium ion lying on the C₂ axis of the formate ion at 2.2 Å from the carbon atom on the oxygen side. The cation-cation and anion-anion interactions are repulsive everywhere, although the potential surface is markedly anisotropic for the HCOO^--HCOO^- interaction.