Hello all,

I'm attempting to apply periodic boundary conditions to a structure having a unit cell dimension based on a TPMS (Triply Periodic Minimal Surface) geometry. To achieve this, I'm using MATLAB to generate node pairs, which are then written to an Abaqus input file using the UNSORTED command.

For instance, I have two sets of nodes: Set_X0 containing nodes 1, 2, and 3, and Set_XN containing nodes 4, 5, and 6. To establish periodicity between these sets, I generate node pairs as follows (which implies that Node 1 in nodesX0 is paired with Node 4 in nodesXN and so on):

**NODE SET GENERATION FOR DUMMY NODES (NODES defined to be used in constraint equations)
*Nset, Nset=dummy
999011, 999012, 999013, 999022, 999023, 999033

**NODE PAIR GENERATION
*Nset, Nset=nodesX0, UNSORTED
1
2
3
*Nset, Nset=nodesXN, UNSORTED
4
5
6

I am defining the equation in the input file as follows:

** PERIODIC BOUNDARY EQUATIONS ARE WRITTEN FOR ENGINEERING SHEAR STRAIN
** THREE EQUATONS FOR 3 DOFs
** 999011, 999012, 999013, 999022, 999023 and 999033 are dummy nodes

*EQUATION
5
nodesX0, 1, -1, nodesXN, 1, 1, 999011, 1, -1, 999012, 1, 0,
999013, 1, 0

*EQUATION
5
nodesX0, 2, -1, nodesXN, 2, 1, 999012, 1, -0.5, 999022, 1, 0,
999023, 1, 0

*EQUATION
5
nodesX0, 3, -1, nodesXN, 3, 1, 999013, 1, -0.5, 999023, 1, 0,
999033, 1, 0

In actuality, I have 481 node pairs between Set_X0 and Set_XN. When I import this input file to Abaqus, it returns the following warning:

WARNING: Abaqus/CAE allows only the first node set in a *EQUATION definition to contain more than one node. Node set NODESXN violates this rule. The equation using this node set will not be imported. This message is only printed once, so please check all other *EQUATION definitions. 

The warning specifically mentions that the node set NODESXN is violating this rule, indicating that it contains more than one node in the *EQUATION definition. Is there a way around this? How, can I create a constraint equation for multiple node pairs? Any guidance would be appreciated.