The band structure of D4h ZnP shows two half-filled bands related by a 90° rotation (Figure ). They are responsible for its metallic conductivity. At the M reciprocal space point, at which the wavefunction is periodic with respect to the square 2⨯2 porphene fragment which contains the antiaromatic eightmembered circuit (whole structure in Figure ), these two bands are exactly degenerate. Breaking the D4h symmetry by introducing BLA lowers the energy of the band concordant with the introduced BLA pattern (making more of it filled) and increases the energy of its rotational partner (making less of it filled), in a direct analogy to symmetry breaking in cyclobutadiene (Figure ). When this Peierls distortion is sufficiently large, a metal to insulator transition occurs (Figure