The electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the self-consistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD including a perturbative expansion for connected triples CCSD(T) levels of theory with a wide range of basis sets. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and Renner-Teller splitting are reported. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state ((X) over tilde (2)Pi) has a linear geometry with r(e)(CH) = 1.0781 Angstrom and r(e)(CSi) = 1.6956 Angstrom. This is in good agreement with the experimental values of r(0)(CH) = 1.0677 Angstrom and r(0)(CSi) = 1.6925 Angstrom, respectively. In the (A) over tilde (2)Sigma (+) state, HCSi is also found to have a linear geometry with r(e)(CH) = 1.0737 Angstrom and r(e)(CSi) = 1.6130 Angstrom at the [CCSD(T)/cc-pVQZ] level, confirming experimental values of r(0)(CH) = 1.0625 Angstrom, r(0)(CSi) = 1.6118 Angstrom, and the observation of C-Si triple bond character. With the same method, the (X) over tilde (2)Pi and (A) over tilde (2)Sigma (+) state C-H stretching vibrational frequencies are predicted to be 3271 and 3319 cm(-1), respectively, for which experimental values are not available. The classical (X) over tilde-(A) over tilde splitting (T-e value) was determined to be 32.6 kcal/mol (1.41 eV, 11 730 cm(-1)) and quantum mechanical splitting (T-0 value) to be 33.5 kcal/mol (1.45 eV, 11726 cm(-1)), which are in excellent agreement with the experimental T-0 value of 11 766.721 cm(-1) (33.64 kcal/mol, 1.459 eV). The linear excited (A) over tilde (2)Sigma (+) state of the molecule has a real degenerate bending vibrational frequency, whereas the ground state ((X) over tilde (2)Pi) is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies. The Renner parameter (epsilon) and average harmonic bending frequency (omega (2)) of the (X) over tilde (2)Pi state are predicted to be epsilon = -0.114 and omega (2) = 518 cm(-1) at the CCSD(T)/cc-pVQZ level of theory. The electronic structure analysis of the ground state showed that the HCSi radical is an A-type Renner-Teller molecule. (C) 2001 American Institute of Physics.