Ab initio electronic structure calculations are performed to support and to help interpret the experimental work reported in the preceding manuscript. The coupled cluster singles and doubles [CCSD(T)] approach, in conjunction with a large basis set, is used to compute spectroscopic constants for the X(1)Sigma(+) and (3)Pi states of ZnO and the X(2)Sigma(+) State of ZnO-. The spectroscopic constants, including the electron affinity (E.A.) are in good agreement with experiment. The ZnO E.A. is significantly larger than that of O, thus relative to the atomic ground-state asymptotes, ZnO- has a larger D-0 than the (1)Sigma(+) state, despite the fact that the extra electron goes into an antibonding orbital. The changes in spectroscopic constants can be understood in terms of the X(1)Sigma(+) formally dissociating to Zn S-1 + O D-1, while the (3)Pi and (2)Sigma(+) states dissociate to Zn S-1(+) O P-3 and Zn S-1 and O- P-2, respectively.