  Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud
formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in
1996, the most important challenge in modeling their atmospheres as become the
understanding of cloud formation and advective mixing. For this purpose, we
have developed radiation hydrodynamic 2D model atmosphere simulations to study
the formation of forsterite dust in presence of advection, condensation, and
sedimentation across the M-L-T VLMs to BDs sequence (Teff = 2800 K to 900 K,
Freytag et al. 2010). We discovered the formation of gravity waves as a driving
mechanism for the formation of clouds in these atmospheres, and derived a rule
for the velocity field versus atmospheric depth and Teff , which is relatively
insensitive to gravity. This rule has been used in the construction of the new
model atmosphere grid, BT-Settl, to determine the microturbulence velocity, the
diffusion coefficient, and the advective mixing of molecules as a function of
depth. This new model grid of atmospheres and synthetic spectra has
beencomputedfor100,000K>Teff >400K,5.5>logg>-0.5,and[M/H]=+0.5to -1.5, and the
reference solar abundances of Asplund et al. (2009). We found that the new
solar abundances allow an improved (close to perfect) reproduction of the
photo- metric and spectroscopic VLMs properties, and, for the first time, a
smooth transition between stellar and substellar regimes -- unlike the
transition between the NextGen models from Hauschildt et al. 1999a,b, and the
AMES-Dusty models from Allard et al. 2001). In the BDs regime, the BT-Settl
models propose an improved explanation for the M-L-T spectral transition. In
this paper, we therefore present the new BT-Settl model atmosphere grid, which
explains the entire transition from the stellar to planetary mass regimes.
