The ground ((X) over tilde (3)Sigma(-)) and first excited triplet ((A) over tilde (3)Pi) electronic states of carbonylsilene or silaketenylidene, SiCO, have been investigated systematically using ab initio electronic structure theory. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were predicted using self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), equation-of-motion (EOM) CCSD, CCSD with perturbative triple excitations [CCSD(T)] methods with a wide range of basis sets. The linear (X) over tilde (3)Sigma(-) ground state of SiCO has a real degenerate bending vibrational frequency, whereas the (A) over tilde (3)Pi state of SiCO is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequency of the (A) over tilde (3)Pi state was evaluated via the EOM-CCSD technique. At the highest level of theory with the largest basis set, cc-pVQZ CCSD(T), the adiabatic (X) over tilde-(A) over tilde splitting without the zero-point vibrational energy (ZPVE) correction (T-e value) was determined to be 68.5 kcal/mol (2.97 eV, 23 900 cm(-1)) and the adiabatic splitting with the ZPVE energy correction (T-0 value) to be 69.0 kcal/mol (2.99 eV, 24 100 cm(-1)), which are in excellent agreement with the experimental T-0 value of 68.78 kcal/mol (2.983 eV, 24 056 cm(-1)). The theoretical ground state harmonic Si-C stretching frequency omega(3) = 564 cm(-1) is much less than the experimental estimate of 800 cm(-1). (C) 2000 American Institute of Physics. [S0021-9606(00)30205-7].