We report the structural and dynamical properties of the ground state ferroelectric P6 3cm structure of hexagonal YMnO 3. The lattice parameters, atomic positions, local magnetic moment of Mn atoms, band gap, and Γ-point phonons are calculated within the local spin-density approximation plus Hubbard U term and the B1-WC hybrid functional. Our results are discussed in comparison to theoretical and experimental values available in the literature. Based on our first-principles calculations, we propose a reassignment of certain modes. We also discuss the relation between the phonon modes of the ferroelectric P6 3cm phase and those of the paraelectric P6 3/mmc phase that condense at the phase transition. © 2011 American Physical Society.