Nonempirical Calculations on Excited States: The Formaldehyde Molecule

A series of calculations on the excited states of formaldehyde using excitation operator techniques are presented. As in ethylene, the effect of $\mathrm{\sigma –\pi }$ interaction on the ${\mathrm{“\pi \to \pi *”(}}^1{A}_1)$ excitation is rather large, decreasing the calculated excitation energy from 14.89 to 12.03 eV and the oscillator strength from 1.01 to 0.30. The coupling has little effect on the corresponding triplet state ${(}^3{A}_1)$ . The next higher approximation reduces the excitation energy to 11.22 eV and the oscillator strength to 0.21. The effect of the coupling on the ${\mathrm{“n\to \pi *”(}}^{\text{1,3}}{A}_2)$ excitations is not as large as that for the ${}^1{A}_1$ state, lowering the excitation energies for both the singlet and triplet by ∼0.5 eV. Similar results were obtained for the ${\mathrm{“\sigma \to \pi *”(}}^{\text{1,3}}{B}_1)$ excitations. Trends are observed in calculations on corresponding states in ethylene and formaldehyde. Numerous one‐electron properties are claculated for the excited states. The results are in moderate agreement with experiment; a major source of error probably arises from the use of an unoptimized, minimum basis set LCAO(STO)–MO–SCF wavefunction.