Since the v i ( r ) are bounded in space and do not overlap, there is an interstitial region between them within which the potential is a constant, usually taken to be zero. In this region, the Schrödinger equation becomes [ ∇ ∇ 2 + α α 2 ] ψ ψ i ( r ) = 0 , where α α = 2 m E / ℏ ℏ . The incoming wave on site i can thus be written in the position representation