  We have generated new, self-consistent spectral and atmosphere models for the
effective temperature range 600 K to 1300 K thought to encompass the known T
dwarfs. For the first time, theoretical models are compared with a {\it family}
of measured T dwarf spectra at wavelengths shortward of $\sim$1.0 micron. By
defining spectral indices and standard colors in the optical and very
near-infrared, we explore the theoretical systematics with \teff, gravity, and
metallicity. We conclude that the short- wavelength range is rich in
diagnostics that complement those in the near-infrared now used for spectral
subtyping. We also conclude that the wings of the Na D and K I (7700\AA)
resonance lines and aggressive rainout of heavy metals (with the resulting
enhancement of the sodium and potassium abundances at altitude) are required to
fit the new data shortward of 1.0 \mic. Furthermore, we find that the water
bands weaken with increasing gravity, that modest decreases in metallicity
enhance the effect in the optical of the sodium and potassium lines, and that
at low \teffs, in a reversal of the normal pattern, optical spectra become
bluer with further decreases in \teff. Moreover, we conclude that T dwarf
subtype is not a function of \teff alone, but that it is a non-trivial function
of gravity and metallicity as well. As do Marley et al. (2001), we see evidence
in early T dwarf atmospheres of a residual effect of clouds. With cloudless
models, we obtain spectral fits to the two late T dwarfs with known parallaxes,
but a residual effect of clouds on the emergent spectra of even late T dwarfs
can not yet be discounted.
