  We show that the origin of electronic transitions of molecular many-body
systems can be revealed by a quantified natural transition orbitals (QNTO)
analysis and the electronic excitations of the total system can be mapped onto
a standard orbitals set of a reference system. We further illustrate QNTO on
molecular systems by studying the origin of electronic transitions of DNA
moiety, thymine and thymidine. This QNTO analysis also allows us to assess the
performance of various functionals used in time-dependent density functional
response theory.
