  We present a new general scheme for calculating the structure and dynamics of
radiation pressure driven, photoionized flows. The new method goes one step
beyond the Sobolev approximation. It involves a numerical solution of the
radiative transfer in absorption lines including the effects of differential
expansion and line interactions such as line locking and blanketing. We also
present a new scheme for calculating the radiation pressure due to trapped line
photons in finite, differentially expanding flows. We compare our results for
the radiation pressure force with those obtained using the Sobolev
approximation and show the limitations of the latter. In particular, we
demonstrate that the Sobolev method gives a poor approximation near
discontinuity surfaces and its neglect of line blanketing can lead to erroneous
results in high velocity flows. We combine the newly calculated radiation
pressure force with self-consistent photoionization and thermal calculations to
study the dynamics and spectral features of broad absorption line flows and
highly ionized gas flows in AGN. A comparison with Sobolev-type calculations
shows that the latter over estimates the flow's terminal velocity and,
conversely, under estimates its opacity. We also show that line locking on
broad emission lines can have a significant effect on the dynamics and spectral
features of AGN flows.
