The thermodynamic properties of Si clusters are calculated using rst principles quantum methods combined with molecular dynamics for simulating the trajectories of clusters. A plane wave basis is used with ab initio pseudopotentials and the local density approximation for determining the electronic energies and forces. Langevin molecular dynamics simulates thermal contact with a constant temperature reservoir. Vibrational spectra, moments of inertia, anharmonic corrections, and free energies are predicted for Si2 through Si5. The translational contribution is based on the ideal gas limit. The rotation contribution is approximated using a classical rigid rotator. Vibrational modes are determined from the dynamical matrix in the harmonic approximation. Corrections due to anharmonicity and coupling between rotational and vibrational modes are t from the molecular dynamics simulations.