  An efficient computational algorithm to implement a local operator approach
to partitioning electronic energy in general molecular systems is presented.
This approach, which rigorously defines the electronic energy on any subsystem
within a molecule, gives a precise meaning to the subsystem ground and excited
electronic energies, which is crucial for investigating electronic energy
transfer from first principles. We apply the technique to the
$9-$(($1-$naphthyl)$-$methyl)-anthracene (A1N) molecule by partitioning A1N
into anthracenyl and CH$_2-$naphthyl groups as subsystems, and examine their
electronic energies and populations for several excited states using
Configuration Interaction Singles method. The implemented approach shows a wide
variety of different behaviors amongst these excited electronic states.
