The open-shell coupled-cluster (CC) method is applied to the excited 2$p^2$ states of Be. With the 2$s^2$ and 2$p^2$ 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 ${}_{}{}^1{}_{}{}^{}\mathrm{S}$ corresponds to the intruder 2s3s configuration, rather than the desired 2$p^2$. The inclusion of 2s3s (but not 3$s^2$) in the model space, which thereby becomes incomplete, improves convergence significantly, and gives energies in very good agreement with values known from other sources.

• Received 1 August 1988

DOI:https://doi.org/10.1103/PhysRevA.38.6013