  We show that the transition origins of electronic excitations identified by
quantified natural transition orbital (QNTO) analysis can be employed to
connect potential energy surfaces (PESs) according to their character across a
widerange of molecular geometries. This is achieved by locating the switching
of transition origins of adiabatic potential surfaces as the geometry changes.
The transition vectors for analysing transition origins are provided by linear
response time-dependent density functional theory (TDDFT) calculations under
the Tamm-Dancoff approximation. We study the photochemical CO ring opening of
oxirane as an example and show that the results corroborate the traditional
Gomer-Noyes mechanism derived experimentally. The knowledge of specific states
for the reaction also agrees well with that given by previous theoretical work
using TDDFT surface-hopping dynamics that was validated by high-quality quantum
Monte Carlo calculations. We also show that QNTO can be useful for considerably
larger and more complex systems: by projecting the excitations to those of a
reference oxirane molecule, the approach is able to identify and analyse
specific excitations of a trans-2,3-diphenyloxirane molecule.
