  Based on a symmetry argument, we study ground states of what we call
MMX-chain compounds, which are the new class of halogen-bridged metal
complexes. Commensurate density-wave solutions of a relevant multi-band
Peierls-Hubbard model are systematically revealed within the Hartree-Fock
approximation. We numerically draw ground-state phase diagrams, where various
novel density-wave states appear.
