  In this work, a generalized force-field methodology for the relaxation of
large moir\'e heterostructures is proposed. The force-field parameters are
optimized to accurately reproduce the structural degrees of freedom of some
computationally manageable cells relaxed using density functional theory. The
parameters can then be used to handle large moir\'e systems. We specialize to
the case of 2H-phased twisted transition-metal dichalcogenide homo- and
heterobilayers using a combination of the Stillinger-Weber intralayer- and the
Kolmogorov-Crespi interlayer-potential. Force-field parameters are developed
for all combinations of MX$_2$ for $\text{M}\in\{\text{Mo},\text{W}\}$ and
$\text{X}\in\{\text{S},\text{Se},\text{Te}\}$. The results show agreement
within 20 meV in terms of band structure between density functional theory and
force-field relaxation. Using the relaxed structures, a simplified and
systematic scheme for the extraction of the interlayer moir\'e potential is
presented for both R- and H-stacked systems. We show that in-plane and
out-of-plane relaxation effects on the moir\'e potential, which is made both
deeper and wider after relaxation, are essential. An interpolation based
methodology for the calculation of the interlayer binding energy is also
proposed. Finally, we show that atomic reconstruction, which is captured by the
force-field method, becomes especially prominent for angles below 4-5$^\circ$,
when there is no mismatch in lattice constant between layers.
