Ab initio quantum mechanical methods are employed in the study of several conformers of silicocene, (C5H5)2Si. Specifically, fully optimized structures are obtained for Cs, C2, D5h, C2v, and D5d isomers at the self-consistent-field (SCF) level of theory. Three different Gaussian basis sets ranging in size from a minimal STO-3G* basis to one of double-ζ plus polarization (DZP) quality were used. The latter set consisted of 210 contracted basis functions. The effects of electron correlation were estimated using second-order perturbation theory (MP2) with the largest basis set. At the SCF DZP level of theory, silicocene is predicted to adopt a low-symmetry bent Csstructure. However, the difference in energy between the low-symmetry (Cs, C2) and C2v conformers is only 2.4 kcal/mol, with the D5dconformer lying 8.8 kcal/mol higher in energy. Electron-correlation effects determined from MP2 energies at the SCF DZP optimized structures favor the higher symmetry species, with a C2v conformer predicted to be 7.5 kcal/mol lower in energy than the Cs/C2 structures. It is shown that basis set augmentation, particularly the inclusion of polarization functions, favors the higher symmetry species. It is concluded that ab initio studies of π-bonded complexes, or sandwich compounds, require at least a DZP basis set to obtain reliable results. These studies do not conclusively determine the symmetry of the minimum energy conformer of silicocene. However, it is clear that the silicocene potential energy surface is flat with respect to motion of the cyclopentadienyl rings about the silicon atom and that if the high-symmetry D5d or D5h conformer is not a minimum, then it will not be significantly higher in energy than the true gas-phase minimum. © 1989, American Chemical Society. All rights reserved.