  Atoms and ions of Lanthanides have multiple opens shells along with an open
$4f^k$ subshell. This paper studies the effect of electron correlation in such
systems and how wave functions can be determined for the accurate prediction of
atomic properties in the case of Ce$^{2+}$ where $k=2$, using the
multireference single- and double-excitation method. An efficient higher-order
method is recommended for more reliable results.
