# Re: CCL:calculation of stress with SHAKE/RATTLE

> how does one calculate the stresses when constraint dynamics is used?
or
> is there a way to seperate the constraint forces as two equal and opposite
> forces so that the stress calculation is simple?(am using the RATTLE
> routine provided in Allen & Tildesley book)
If you have only pair distance constraints, then the constraint forces
can be decomposed in such a way. In fact, just look at the derivation
of SHAKE and RATTLE: the constraint forces are written as a Lagrange
multiplier times the pair distance. The Lagrange multipliers can be
calculated explicitly during the SHAKE/RATTLE iterations.
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