Excitation energies in Be: A comparison of multiconfigurational linear response and full configuration interaction calculations

Using a (9s9p5d) contracted GTO basis we have calculated low-lying excitation energies of singlet and triplet symmetry for the Be atom using Δfull CI, ACI (1s) with double occupancy in the 1s orbital, multiconfiguration linear response (MCLR), and ΔMCSCF approaches. The Δfull CI results agree very closely with the experimental excitation energies except for higher excitations where obvious basis set defects occur. The MCLR calculations shows that with an adequately chosen MCSCF reference state the MCLR calculation is capable of mimicking the Δfull CI results. The MCLR results are closer to the Δfull CI results than the ΔCI(1s). The ΔMCSCF excitation energies show that this approach can only be used with extreme care to determine excitation energies.