Bond-length alternation
= Bond-length alternation
{title2=$\beta_S-\beta_L$}
Alternation between shorter and longer <chemical bonds> along a <conjugated system>. In the <Hückel method>, unequal nearest-neighbour couplings split the frontier <molecular orbitals>. An electronic <energy> gain must be compared at a stated constraint on the mean coupling and against the geometric distortion cost.