where the term in the square bracket is constant if there is no heteroatom in the system. The absolute value of the ground state expectation value |⟨ D⟩| = |⟨g| D|g⟩| is a direct measure of how much the particle-hole symmetry is broken, which in turn impacts the optical transition probabilities. In the following, we will see how |⟨ D⟩| changes as a function of the aza nitrogen transition allowed by point group symmetry but forbidden by particle-hole symmetry.