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One of the main mechanisms that could drive mass outflows in AGNs isradiation pressure due to spectral lines. Although straightforward tounderstand, the actual magnitude of the radiation force is challenging tocompute because the force depends on the physical conditions in the gas, andthe strength, spectral energy distribution (SED), and geometry of the radiationfield. We present results from our photoionization and radiation transfercalculations of the force multiplier, $M(\xi,t)$, using the same radiationfield to compute the gas photoionization and thermal balance. We assume low gasdensity ($n = 10^4~\rm{cm^{-3}}$) and column density ($N \leq 10^{17}~\rm{cm^{-2}}$), a Boltzmann distribution for the level populations, and the Sobolevapproximation. Here, we describe results for two SEDs corresponding to anunobscured and obscured AGN in NGC 5548. Our main results are the following: 1)although $M(\xi,t)$ starts to decrease with $\xi$ for $\xi \gtrsim 1$ as shownby others, this decrease in our calculations is relatively gradual and could benon-monotonic as $M(\xi,t)$ can increase by a factor of few for $\xi \approx10-1000$; 2) at these same $\xi$ for which the multiplier is higher than inprevious calculations, the gas is thermally unstable by the isobaric criterion;3) non-LTE effects reduce $M(t,\xi)$ by over two orders of magnitude for $\xi\gtrsim 100$. The dynamical consequence of result (1) is that line driving canbe important for $\xi$ as high as $1000$ when the LTE approximation holds,while result (2) provides a natural cloud formation mechanism that may accountfor the existence of narrow line regions. Result (3) suggests that line drivingmay not be important for $\xi\gtrsim100$ in tenuous plasma.

Photoionization calculations of the radiation force due to spectral lines in AGNs