Quantum defects for berylliumsize-2Ifrom the poles of the multichannelTmatrix

Quantum defects are determined for the beryllium ³S, ³P ^∘{}, and ¹P ^∘{} Rydberg series below the first ionization threshold by searching for the poles of the multichannel Schwinger T matrix along the negative real-energy axis. The ab initio calculations are performed using static-exchange target functions for the ion cores. The results show increasing accuracy as the principal quantum number increases reflecting the fact that this method essentially treats bound states as negative-energy states scattering off an ion core. Small fixed grids and small fixed basis sets are sufficient to describe each Rydberg series of a particular symmetry. The long-range behavior is enforced by using a closed-channel (i.e., negative-energy) Coulomb Green's function. Channel wave functions with the correct asymptotic form can be easily determined using this method.