Analytical potential energy function for the ground state X1Σ+ of LaCl

The equilibrium geometry, harmonic frequency and bond dissociation energy of lanthanum monochloride have been calculated at B3LYP, MP2, QCISD(T) levels with the energy-consistent Relativistic Effective Core Potentials. The possible electronic state and reasonable dissociation limit are determined based on atomic and molecular reaction statics. Potential energy curve scans for the ground state X1Σ+ have been performed with B3LYP and QCISD(T), since they exhibit better performance than other methods in calculations of bond energy. We find that the potential energy calculated with QCISD(T) is about 0.5 eV larger than the bond dissociation energy, when the diatomic distance is as large as 0.8 nm. The problem that single-reference ab initio methods do not meet the dissociation limit during calculations of lanthanide heavy-metal elements is analyzed. We propose the calculation scheme to derive the analytical Murrell–Sorbie potential energy function and Dunham expansion at the equilibrium position. Spectroscopic constants obtained by the standard Dunham treatment are in good agreement with the results of rotational analyses on spectroscopic experiments. The analytical function is of much realistic importance, since it is possible to be applied to predict the transitional structure and study the reaction dynamic process. [Copyright &y& Elsevier]