Multiconfigurational perturbation theory with level shift — the Cr2 potential revisited

A level shift technique is suggested for removal of intruder states in multiconfigurational second-order perturbation theory (CASPT2). The first-order wavefunction is first calculated with a level shift parameter large enough to remove the intruder states. The effect of the level shift on the second-order energy is removed by a back correction technique (the LS correction). It is shown that intruder states are removed with little effect on the remaining part of the correlation energy. New potential curves have been computed for the X 1Σg+ and the á 3Σu+ states of Cr2 using large basis sets (ANO: 8s7p6d4f2g) and accounting for relativistic effects, 3s and 3p correlation, and basis set superposition effects. The computed spectroscopic constants (experimental values in parentheses) for the X 1Σg+ state are re = 1.69(1.68) Å, ΔG12 = 535(452) cm−1, D0 = 1.54(1.44) eV. The corresponding values for á 3Σu+ are re = 1.64(1.65) Å, ΔG12 = 667(574) cm−1, Te = 1.79(1.76) eV.