  Contact dynamics (CD) is a powerful method to solve the dynamics of large
systems of colliding rigid bodies. CD can be computationally more efficient
than classical penalty-based discrete element methods (DEM) for simulating
contact between stiff materials such as rock, glass, or engineering metals.
However, by idealizing bodies as perfectly rigid, contact forces computed by CD
can be non-unique due to indeterminacy in the contact network, which is a
common occurence in dense granular flows. We propose a CD method that is
designed to identify only the unique set of contact forces that would be
predicted by a soft particle method, such as DEM, in the limit of large
stiffness. The method involves applying an elastic compatibility condition to
the contact forces, which maintains no-penetration constraints but filters out
force distributions that could not have arisen from stiff elastic contacts. The
method can be used as a post-processing step that could be integrated into
existing CD codes with minimal effort. We demonstrate its efficacy in a variety
of indeterminate problems, including some involving multiple materials,
non-spherical shapes, and nonlinear contact constitutive laws.
