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 ( { T } _ { eff } = 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 5891,5897Å Na I D and \lambda 7687,7701Å 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 .