  A curious behavior of electron correlation energy is explored. Namely, the
correlation energy is the energy that tends to drive the system toward that of
the uniform electron gas. As such, the energy assumes its maximum value when a
gradient of density is zero. As the gradient increases, the energy is
diminished by a gradient suppressing factor, designed to attenuate the energy
from its maximum value similar to the shape of a bell curve. Based on this
behavior, we constructed a very simple mathematical formula that predicted the
correlation energy of atoms and molecules. Combined with our proposed exchange
energy functional, we calculated the correlation energies, the total energies,
and the ionization energies of test atoms and molecules; and despite the unique
simplicities, the functionals' accuracies are in the top tier performance,
competitive to the B3LYP, BLYP, PBE, TPSS, and M11. Therefore, we propose that,
as guided by the simplicities and supported by the accuracies, the correlation
energy is the energy that locally tends to drive the system toward the uniform
electron gas.
