  Local-density-approximation calculations are used to show that the
metal-metal distance along the chains controls the charge-density wave (CDW) in
halogen-bridged transition-metal linear-chain (MX) compounds. The strength of
the CDW can be understood in terms of a two band Su-Schrieffer-Heeger model if
a hard-core ion-ion repulsion potential is also added. We predict a
second-order phase transition from an insulating to a semi-metallic ground
state and explain trends in dimerization, bond-length ratios, band gaps, and
Raman breathing modes in terms of the metal-metal distance. Three "ps" figures
are included at the end of the file.
