  We present an extension of the spin-adapted configuration-interaction method
for the computation of four electrons in a quasi two-dimensional quantum dot.
By a group-theoretical decomposition of the basis set and working with relative
and center-of-mass coordinates we obtain an analytical identification of all
spurious center-of-mass states of the Coulomb-interacting electrons. We find a
substantial reduction in the basis set used for numerical computations. At the
same time we increase the accuracy compared to the standard spin-adapted
configuration-interaction method (SACI) due to the absence of distortions
caused by an unbalanced cut-off of center-of-mass excitations.
