Structure and stability of hydrogen clusters up to H₂₁⁺

Low-lying stationary points on the H₂₁⁺ potential energy hypersurface have been determined through density functional theory (DFT) calculations with the recently proposed functional B3(H) especially designed for hydrogen systems. Calculations have been carried out also for conformers of H₁₇⁺ and H₁₉⁺ leading to fully optimized structures and energies. Only one structure has been found as a true minimum for H₂₁⁺, which exhibits a high D_3h symmetry. The total energies of the two structures of isomers of H₁₇⁺ differ by less than 0.1 kJ mol^-1. Vibrational frequencies and infrared intensities for all stationary points were predicted and compared to frequencies previously obtained for smaller clusters.