  Three new measures of relative electron motion are introduced: equimomentum,
antimomentum, and momentum-balance. The equimomentum is the probability that
two electrons have the exact same momentum, whereas the antimomentum is the
probability their momenta are the exact opposite. Momentum-balance (MB) is the
difference between the equimomentum and antimomentum, and therefore indicates
if equal or opposite momentum is more probably in a system of electrons. The
equimomentum, antimomentum and MB densities are also introduced, which are the
local contribution to each quantity. The MB and MB density of the
extrapolated-Full Configuration Interaction wave functions of atoms of the
first three rows of the periodic table are analyzed, with a particular focus on
contrasting the correlated motion of electrons with opposite and parallel spin.
Coulomb correlation between opposite-spin electrons leads to a higher
probability of equimomentum, whereas Fermi correlation between parallel-spin
electrons leads to a higher probability of antimomentum. The local contribution
to MB, given an electron is present, is a minimum at the nucleus and generally
increases as the distance from the nucleus increases. There are also
interesting similarities between the effects of Fermi correlation and Coulomb
correlation (of opposite-spin electrons) on MB.
