Intruder states and incomplete model spaces in multireference coupled-cluster theory: The 2p2 states of Be

The open-shell coupled-cluster (CC) method is applied to the excited 2p2 states of Be. With the 2s2 and 2p2 configurations included in the model (P) space, the CC equations prove very difficult to converge. When they do converge, very large (>5) excitation amplitudes are observed, and the second S1 corresponds to the intruder 2s3s configuration, rather than the desired 2p2. The inclusion of 2s3s (but not 3s2) in the model space, which thereby becomes incomplete, improves convergence significantly, and gives energies in very good agreement with values known from other sources.