At low temperature, or in system with a large degree of structural disorder (such as fully amorphous systems), electrons cannot access delocalized states. In such a system, electrons can only travel by tunnelling for one site to another, in a process called variable range hopping. In the original theory of variable range hopping, as developed by Mott and Davis, the probability P i j , of an electron hopping from one site i , to another site j , depends on their separation in space r i j , and their separation in energy Δ Δ E i j .