Investigation of analytic potential energy function, harmonic frequency and vibrational levels for the 23Σ+g state of spin-aligned dimer 7Li2

Abstract

Using the symmetry adapted cluster/symmetry adapted cluster-configuration interaction(SAC/SAC-CI) method in Gaussian03 program package, the equilibrium geometry of the 23Σ+g state of spin-aligned dimer 7Li2 is calculated at a number of basis sets. At the same time, the single-point energy scanning calculation is also made at each basis set near the equilibrium internuclear separation obtained by the geometry optimization so as to attain the more accurate result. A disagreement between the result obtained by the geometry optimization and that obtained by the single-point energy scanning calculation is found. Our analysis shows that the result obtained by the single-point energy scanning calculation should be more reasonable. We drew the conclusion that the basis sets 6-311++G(3df,3pd), 6-311++G(2df,2pd) and 6-311++G(2df,pd) are the most suitable ones for the 23Σ+g state calculation. The complete potential energy curve is further scanned at SAC-CI/6-311++G(3df,3pd) level of theory for the state over the internuclear separation range from 2.5a0 to 37a0, then a least squares fit to the Murrell-Sorbie function is made, at last the harmonic frequency is calculated, which is in good agreement with other theoretical results. At the same time, the same calculations are made for the ground state for comparison. In addition, we have also calculated the vibrational levels and the classical turning points.