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The applicability of multicomponent quantum mechanical (MC_QM) approach was assessed by calculating the H/D isotope effect on the electronic absorption spectra of small molecules. The calculated excitation energy of the water molecule increased upon H/D substitution, a trend consistent with the experimentally observed peak shift in the absorption cross-section. This increase in excitation energy was primarily attributed to changes in the highest occupied molecular orbital, driven by a reduction in nuclear quantum fluctuations due to isotope substitution. Similar trends were observed in the absorption spectra of ethylene, hexatriene, and biphenyl, reinforcing the generality of these findings. These results suggest that MC_QM is an efficient method for evaluating H/D isotope effects on ultraviolet–visible spectra without requiring vibrational analysis. Furthermore, the approach provides a clear connection between spectral changes and alterations in molecular orbitals induced by isotope substitution.Graphical Abstract