The quantized internal energy of a molecule in its electronic ground or excited state can be approximated with sufficient accuracy for analytical purposes by: \[E_{\rm{int}}=E_{\rm{el}}+E_{\rm{vib}}+E_{\rm{rot}}\] where \(E_{\mathrm{el}}\) is the electronic, \(E_{\mathrm{vib}}\) the vibrational and \(E_{\mathrm{rot}}\) the rotational energy, respectively.