  The enhancement of the parity-violating energy difference (PVED) by
electronic excitation is studied for H$_2X_2$ ($X =$ O, S, Se, Te), CHFClBr,
CHFClI, and CHFBrI. To clarify the enhancement mechanism, the dihedral angle
dependence of the PVED of H$_2X_2$ in excited states is studied. If the
contribution from the highest occupied molecular orbital (HOMO) to the PVED in
the ground state is larger than the sum of those from all occupied orbitals,
the PVED in the first excited state has a much larger value compared to the
ground state due to cancellation breaking among valence orbital contributions.
This enhancement is named cancellation breaking enhancement. The PVED
enhancement is also studied for CHFClBr, CHFClI, and CHFBrI in excited states,
and the cancellation breaking enhancement is confirmed. When the PVED
contribution from the HOMO is larger than any other contribution, the
cancellation breaking enhancement hypothesis provides the estimate of PVED in
the first excited state from the HOMO contribution.
