The 2×1-AlMD-2H structure is not only the only stable structure that does not comply with the ECM. It is also the only structure that gives rise to a metallic band structure, see Fig. . It features one-dimensional Al-In atomic wires along the [110] direction, which are partially H decorated. The Al-In metal bonds result in a quasione-dimensional electron band, which disperses along the atomic wire direction and pins the Fermi energy at midgap position. Similar electronic properties are calculated for the 2×2-2InMD-2H structure. Here, an In-In atomic wire extends along the [110] direction and gives rise to two strongly dispersive electron bands. These two bands, one occupied and one empty, are separated by a small band gap of about 0.2 eV and pin the Fermi energy at mid-gap position. However, these two structures are stable only in a very small window of preparation conditions, where the cation-rich surface is exposed to hydrogen. The β2-2×4, the 2×2-4P-6H, and, in particular the 2×2-2D-2H structures are far more prominent in the calculated surface phase diagram. They correspond to P-rich surfaces without hydrogen, in extremely hydrogenrich, and at intermediate conditions, respectively. In case of the 2×2-2D-2H structure, an occupied bound surface state is observed that extends slightly above the bulk valence band maximum (VBM), see Fig. . This state is mainly related to the dangling bonds on the up atom of the phosphorus dimer. Exposing the P-rich AlInP(001) surface to very hydrogen-rich conditions leads to the 2×2-4P-6H structure and removes essentially all surface states from the band gap region. In fact, the only remaining surface state occurs at about 0.2 eV below the bulk VBM and corresponds to dangling bonds at under-coordinated surface P atoms. In case of the hydrogen-free β2-2×4 surface, an occupied surface state is observed at about 0.2 eV above the bulk VBM. This only weakly dispersive feature corresponds to an anti-bonding π * combination of p z orbitals localized at the third-layer P dimer. Additionally, an empty surface state extends slightly into the region of the bulk band gap. It is related to empty dangling bonds located at the three-fold coordinated secondlayer cations. It is mainly In localized, but also has some Al contribution.