Secular Equations and Secular Determinants
Equations 12 and 13 form the secular equations:
(14)
These equation have a "trivial" and useless solution c1=
c2 = 0. The condition
that there should exist a nontrivial solution of these equations is that the
secular determinant should be zero:
(15)
Everything in this equation is a known number except
E.
The equation is therefore an equation for E. In the present case
where the molecular orbital was a linear combination of just two atomic
orbitals it is a quadratic equation and has two solutions for E.
These two values of E are the molecular orbital energies. We always get the same number of molecular
orbitals as atomic orbitals we start with.
Finding the MO energies by
multiplying out the secular determinant