An electron correlation has been examined in detail for the 1s2 -state of He and comparisons are
made with the ground state of Li+, Be2+ and B3+. An expression has been obtained for the partial Coulomb
hole associated with any pair of occupied HF spin orbital for many electron systems. The required
partitioning of the correlated second order density matrix was achieved here, up to and including the paircorrelation
effects. Partial coulomb holes were determined in momentum space. The concept of partial
coulomb holes and their collective presentation as surfaces has been demonstrated to be particularly
informative. These surfaces enabled us to interpret correlation effects in momentum space providing a
rationalization of the correlation mechanisms in each state.
Key words: Correlation effects, He-like ions, Momentum space.