Rigid diaphragm master node questions

[ffilippi]

I noticed that instead of creating an additional node to act as the master node for a rigid diaphragm, using one of the diaphragm constrained nodes as the master node does not influence the results (modal analysis).
Are there any dangers with this? Is there any reason why creating a "virtual" node is recommended?

For example to constrain nodes 5,6,7,8:
Instead of:
node 9 $X9 $Y9 $Z9; (the added virtual node)
fix 9 0 0 1 1 1 0 ;
rigidDiaphragm $perpDirn 9 5 6 7 8;

Just have:
rigidDiaphragm $perpDirn 5 6 7 8;

Thank you.

[selimgunay]

A virtual node would be useful if you lump all the masses to a node at the center of mass. If you assign masses to individual nodes, you don't need the extra virtual node.

[ffilippi]

selimgunay wrote:
> A virtual node would be useful if you lump all the masses to a node at the
> center of mass. If you assign masses to individual nodes, you don't need
> the extra virtual node.

Thank you very much for your reply.