  We have generated an extensive new suite of massive giant planet atmosphere
models and used it to obtain fits to photometric data for the planets HR 8799b,
c, and d. We consider a wide range of cloudy and cloud-free models. The cloudy
models incorporate different geometrical and optical thicknesses, modal
particle sizes, and metallicities. For each planet and set of cloud parameters,
we explore grids in gravity and effective temperature, with which we determine
constraints on the planet's mass and age. Our new models yield statistically
significant fits to the data, and conclusively confirm that the HR 8799 planets
have much thicker clouds than those required to explain data for typical L and
T dwarfs. Both models with 1) physically thick forsterite clouds and a
60-micron modal particle size and 2) clouds made of 1 micron-sized pure iron
droplets and 1% supersaturation fit the data. Current data are insufficient to
accurately constrain the microscopic cloud properties, such as composition and
particle size. The range of best-estimated masses for HR 8799b, HR 8799c, and
HR 8799d conservatively span 2-12 M_J, 6-13 M_J, and 3-11 M_J, respectively and
imply coeval ages between ~10 and ~150 Myr, consistent with previously reported
stellar age. The best-fit temperatures and gravities are slightly lower than
values obtained by Currie et al. (2011) using even thicker cloud models.
Finally, we use these models to predict the near-to-mid IR colors of soon-to-be
imaged planets. Our models predict that planet-mass objects follow a locus in
some near-to-mid IR color-magnitude diagrams that is clearly separable from the
standard L/T dwarf locus for field brown dwarfs.
