1. The problem statement, all variables and given/known data I'm considering a molecule made by three atoms, each a vertex of an equilateral triangle. Each atom has a covalent bond with its neighbours, sharing their only valence electron. I must estimate the energy gain when creating the molecule, using tight binding theory. 2. Relevant equations How much is the energy gain when the molecule is created? 3. The attempt at a solution I used tight binding theory and named each atom's orbital |1>,|2> and |3>. Assuming that [tex] <i|H|j>=-t (hopping) [/tex] if [tex] i\neq j [/tex] and [tex] <i|H|j>= \epsilon _0 [/tex] if [tex] i=j [/tex]. I wrote my 3x3 matrix and calculated the eigenvalues which are-2t, t and t. The solutions tell that the energy gain is -2t-2t+t = -3t and I don't understand why.