quantized internal energy

The quantized internal energy of a molecule in its electronic ground or excited state can be approximated with sufficient accuracy for analytical purposes by: \[E_{\text{int}}=E_{\text{el}}+E_{\text{vib}}+E_{\text{rot}}\] where E el is the electronic, E vib the vibrational and E rot the rotational energy, respectively.
PAC, 1988, 60, 1449. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - VII. Molecular absorption spectroscopy, ultraviolet and visible (UV/VIS) (Recommendations 1988)' on page 1452 (https://doi.org/10.1351/pac198860091449)