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