  We present opacity sampling model atmospheres, synthetic spectra and colors
for brown dwarfs and very low mass stars in two limiting case of dust grain
formation: 1) inefficient gravitational settling i.e. the dust is distributed
according to the chemical equilibrium predictions, 2) efficient gravitational
settling i.e. the dust forms and depletes refractory elements from the gas, but
their opacity does not affect the thermal structure. The models include the
formation of over 600 gas phase species, and 1000 liquids and crystals, and the
opacities of 30 different types of grains including corundum (Al$_2$O$_3$), the
magnesium aluminum spinel MgAl$_2$O$_4$, iron, enstatite (MgSiO$_3$),
forsterite (Mg$_2$SiO$_4$), amorphous carbon, SiC, and a number of calcium
silicates. The models extend from the beginning of the grain formation regime
well into the condensation regime of water ice ($\teff= 3000 - 100$ K) and
encompasses the range of $\log g= 2.5 - 6.0$ at solar metallicity.
  We find that silicate dust grains can form abundantly in the outer
atmospheric layers of red and brown dwarfs with spectral type later than M8.
The greenhouse effects of dust opacities provide a natural explanation for the
peculiarly red spectroscopic distribution of the latest M dwarfs and young
brown dwarfs. The grainless (Cond) models on the other hand, correspond closely
to methane brown dwarfs such as Gliese 229B. We also recover that the
$\lambda$5891,5897\AA Na I D and $\lambda$7687,7701\AA K I resonance doublets
plays a critical role in T dwarfs where their red wing define the
pseudo-continuum from the $I$ to the $Z$ bandpass.
