The CIPT2 method: Coupling of multi-reference configuration interaction and multi-reference perturbation theory. Application to the chromium dimer

The potential energy function of Cr-2(X(1)Sigma(g)(+)) is computed using the MRCI + Q (multi-reference configuration interaction with Davidson correction) and CASPT2 (complete active space second-order multi-reference perturbation theory) methods and very large basis sets. In addition, a new hybrid method denoted CIPT2, in which excitations solely from the active space are treated by MRCI and the remaining ones by perturbation theory, is proposed and applied. These methods are used to analyse valence and core correlation effects on the potential energy function. MRCI calculations which fully include correlation of the 3p, 3d, and 4s electrons yield excellent agreement with experimental values for the equilibrium distance R-e and the harmonic vibrational constant omega(e). Correlation of the (3p) orbitals is found to be very important. At the MRCI+Q level this reduces R-e by 0.08 Angstrom and increases omega(e) by about 200 cm(-1). These effects are not correctly reproduced in CASPT2 and CIPT2. It is shown that in CASPT2 the valence correlation effects are significantly overestimated and the distance dependence of the core correlation is incorrect. The basis set limit of the dissociation energies for MRCI + Q and CASPT 2 is estimated to be 1.3 eV and 1.9 eV, respectively. The remaining error (approximate to 0.2 - 0.3 eV) of the MRCI + Q dissociation energy is attributed to unlinked cluster and higher excitation effects, which are only approximately accounted for by the Davidson correction.