I have built a DEM model that leverages the new functional capabilities in Abaqus 2016 to simulate tire-ground interactions. However, I get errors that seem strange to me. I defined a node set to represent the DEM elements that uses PD3D, which has 6-dof. But the error message is imploring me to constrain the all dof of the PD3D elements.  This is the error message:

***ERROR: Each of the nodes listed below has activated rotational degrees-of-freedom without rotary inertia, but not all rotational degrees of freedom are constrained at the node. Abaqus/Explicit now requires a nonzero inertia at all nodes with activated rotational degrees-of-freedom that are not tie nodes on a rigid body unless rotational constraints are applied with the *BOUNDARY option (see the Abaqus Analysis Users Manual). This situation typically occurs at nodes surrounded only by elements with no density (e.g. spring, dashpot, connector or fluid elements). To remedy this error, either add rotational constraints to the nodes with the *BOUNDARY option or inertia using the *ROTARY INERTIA option, whichever is appropriate to obtain the desired response. A node set named "ErrNodeRotNodeNoRotInert" containing the problematic nodes is created to help the user locate the error and modify the input definition.

Deformable Nodes Without Inertia: 4000, 4001, 4002, 4003, 4004, 4005, 4006, 4007 ...

Any help?

Eric