about the fix rigid

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1 
     I have been confused about the command "fix rigid".In our work,the indenter configeration is hemisphere .If we employ tersoff potential,the indenter can keep vertical on the direction of loading force all the time .However,when change another potential,namely airebo potential ,the indenter can't keep vertical,it's slant if the load force too small.If the force is large enough,the indenter can keep balance."fix  5 forceitem rigid single force 1 off off on" is my command,forceitem refers to the indenter.

How to make the indenter not lean?

Best wishes,
xianglang
2

Rigid bodies can rotate as well as translate. You can
turn off torque on a rigid body (see the fix rigid doc page)
and thus prevent rotation. But you can likely not use
fix rigid at all and simply use fix ave/force in the vertical
direction. See examples/flow/in.poiss for an example
of a wall that is treated effectively rigid in that manner,
but it moves up and down.

Steve