Equilibrium geometries of two isomers of cyclic SiC3 are determined by both large scale ab initio calculation and a procedure involving the use of experimental data from microwave spectroscopy and calculated harmonic and cubic force fields. Internuclear distances corresponding to structures obtained by both procedures agree to better than 0.003 Angstrom in all cases, allowing for precise recommendations of equilibrium structures. Rather large residual inertial defects obtained from moments of inertia based on rotational constants adjusted for effects of vibration-rotation interaction were found to be reduced significantly by inclusion of electronic contributions as estimated from calculations of rotational g-tensors. (C) 2001 American Institute of Physics.