Electronic structure

Electrons are the “glue” that holds the nuclei together in the chemical bonds of
molecules and ions. It  is the nuclei’s positive charges that bind the electrons to
the nuclei. Electronic structure theory deals with the quantum states of the electrons, usually within the Born- Oppenheimer approximation .It also addresses the forces that the electrons’ presence creates on the nuclei; it is these forces that determine the geometries and energies of various stable structures of the molecule as well as transition states connecting these stable structures. Because there are ground and excited
electronic states, each of which has different electronic properties, there are different
stable-structure and transition-state geometries for each such electronic state. Electronic
structure theory deals with all of these states, their nuclear structures, and the
spectroscopies.

Reference
http://simons.hec.utah.edu/NewUndergradBook/Chapter6