We show how calculated ionization potentials and electron affinities of doped diamond nanoparticles can be used to predict the donor and acceptor excitation energies of doped n-type and p-type semiconducting bulk diamond. The method uses good quality quantum chemical calculations on small clusters of diamond and doped diamond. Excitation energies are calculated based on differences in total energies of neutral and positively or negatively charged clusters. When charges are free in the bulk, as for an electron associated with substitutional B-, a simple particle-in-a-box kinetic energy of cluster confinement is taken into account. Optical and thermal excitation energies are calculated for dopants using the B3LYP hybrid density functional method with the 6-31G basis set and substituted C44H42 nanocrystals in the bulk structure and with structure optimization around the defects. Based on eight experimentally determined values, the errors of this method are 0.6 eV and less. Calculations at this level allow screening for potential shallow donor and shallow acceptor defects. A classical approach to estimating the cluster confinement energy based on charging the cluster in a polarized dielectric continuum is tested and shown to be less successful.