  Charm final states in deep inelastic scattering constitute $\sim 25%$ of the
inclusive cross-section at small $x$ as measured at HERA. These data can reveal
important information on the charm and gluon structure of the nucleon if they
are interpreted in a consistent perturbative QCD framework which is valid over
the entire energy range from threshold to the high energy limit. We describe in
detail how this can be carried out order-by-order in PQCD in the generalized
\msbar formalism of Collins (generally known as the ACOT approach), and
demonstrate the inherent smooth transition from the 3-flavor to the 4-flavor
scheme in a complete order $\alpha_s$ calculation, using a Monte Carlo
implementation of this formalism. This calculation is accurate to the same
order as the conventional NLO $F_2$ calculation in the limit $\frac{Q}{m_c} >>
1$. It includes the resummed large logarithm contributions of the 3-flavor
scheme (generally known in this context as the fixed-flavor-number or FFN
scheme) to all orders of $\alpha_s\ln(m_c^2/Q^2)$. For the inclusive structure
function, comparison with recent HERA data and the existing FFN calculation
reveals that the relatively simple order-$\alpha_s$ (NLO) 4-flavor ($m_c \neq
0$) calculation can, in practice, be extended to rather low energy scales,
yielding good agreement with data over the full measured $Q^2$ range. The Monte
Carlo implementation also allows the calculation of differential distributions
with relevant kinematic cuts. Comparisons with available HERA data show
qualitative agreement; however, they also indicate the need to extend the
calculation to the next order to obtain better description of the differential
distributions.
