To find the various energy corrections we take advantage of the turnover rule and orthonormality conditions by left multiplying Eqns 16 - 19 by .
.
This is why the factor of 
was omitted from Eqns
16 - 19.
we take advantage of
the turnover rule and orthonormality conditions by left multiplying
Eqns 16 - 19 by 
.