  We present calculations of the dynamics of highly ionized gas clouds that are
confined by external pressure, and are ionized by AGN continuum. We focus on
the gas that is seen in absorption in the X-ray spectrum of many AGN and show
that such gas can reach hydrostatic equilibrium under various conditions. The
principal conclusion is that the clouds can be accelerated to high velocities
by the central X-ray source. The dynamical problem can be reduced to the
calculation of a single parameter, the average force multiplier, <M>. The
typical value of <M> is ~10 suggesting that radiation pressure acceleration by
X-rays is efficient for L/L_Edd>0.1. The terminal velocity scales with the
escape velocity at the base of the flow and can exceed it by a large factor.
The typical velocity for a HIG flow that originates at R=1e17 cm in a source
with L_x=1e44 erg/s is ~1000 km/s, i.e. similar to the velocities observed in
several X-ray and UV absorption systems.
  Highly ionized AGN clouds are driven mainly by bound-free absorption and
bound-bound processes are less important unless the lines are significantly
broadened or the column density is very small. Pressure laws that result in
constant or outward decreasing ionization parameters are most effective in
accelerating the flow.
