  Mercury has perhaps the strangest behavior of any of the metals. Although the
other metals in column IIB have an $hcp$ ground state, mercury's ground state
is the body centered tetragonal $\beta$Hg phase. The most common phase of
mercury is the rhombohedral $\alpha$Hg phase, which is stable from 79K to the
melting point and meta-stable below 79K. Another rhombohedral phase,
$\gamma$Hg, is believed to exist at low temperatures. First-principles
calculations are used to study the energetics of the various phases of mercury.
Even when partial spin-orbit effects are included, the calculations indicate
that the hexagonal close packed structure is the ground state. It is suggested
that a better treatment of the spin-orbit interaction might alter this result.
