The results of ab initio calculations of the vibronic spectra involving both lowest-lying states X2A and A2A of the ketenyl radical HCCO are presented. Potential energy surfaces are computed by means of the coupled cluster method with perturbative triple corrections [CCSD(T)]. A recently developed ab initio approach for the treatment of the Renner–Teller effect in tetra-atomic asymmetric molecules is applied to calculate the vibronic energy levels.

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