Calculated properties of P2, P4, and of closed-shell clusters up to P18

The thermodynamic stabilities of the most stable even-membered and electrically neutral molecules (clusters) of phosphorus P2 through P18 are calculated in high accuracy. The larger clusters Pn, n≳4, turn out to be unstable at elevated temperatures, but are thermodynamically stable with respect to decay into P4 at low temperatures. To aid an experimental identification of such clusters, calculated vibrational frequencies, ionization potentials, and electronic excitation energies are supplied. As a side result we find the equilibrium bond length in P4 to be substantially shorter (below 220 pm) than the established experimental value r0=222.28±0.05 pm.