metallic_region1 = BoxRegion(
    1*Volt,
    xmin=0*Angstrom, xmax=41.439*Angstrom,
    ymin=0*Angstrom, ymax=0.5*Angstrom,
    zmin=5*Angstrom, zmax=15.*Angstrom
    )

metallic_region2 = metallic_region1(
    value=-1*Volt,
    ymin=10*Angstrom, ymax=12*Angstrom
    )