  A hot central star illuminating the surrounding ionized H II region usually
produces very rich atomic spectra resulting from basic atomic processes:
photoionization, electron-ion recombination, bound-bound radiative transitions,
and collisional excitation of ions. Precise diagnostics of nebular spectra
depend on accurate atomic parameters for these processes. Latest developments
in theoretical computations are described, especially under two international
collaborations known as the Opacity Project (OP) and the Iron Project (IP),
that have yielded accurate and large-scale data for photoionization cross
sections, transition probabilities, and collision strengths for electron impact
excitation of most astrophysically abundant ions. As an extension of the two
projects, a self-consistent and unified theoretical treatment of
photoionization and electron-ion recombination has been developed where both
the radiative and the dielectronic recombination processes are considered in an
unified manner. Results from the Ohio State atomic-astrophysics group, and from
the OP and IP collaborations, are presented. A description of the electronic
web-interactive database, TIPTOPBASE, with the OP and the IP data, and a
compilation of recommended data for effective collision strengths, is given.
