Question about "fix deform"

According to the manual,
“For solids or liquids, note that when one dimension of the box is expanded via fix deform (i.e. tensile strain), it
may be physically undesirable to hold the other 2 box lengths constant (unspecified by fix deform) since that
implies a density change. Using the volume style for those 2 dimensions to keep the box volume constant may
make more physical sense, but may also not be correct for materials and potentials whose Poisson ratio is not 0.5.
An alternative is to use fix npt aniso with zero applied pressure on those 2 dimensions, so that they respond to the
tensile strain dynamically.”

Does it mean Poisson ratio should be exact 0.5 to use “volume” option with “fix deform”?
For example, if I have a material whose Poisson ratio is 0.3 or 0.7, I cannot use “volume” option. Is that right?



You can use the volume option with any system. The Q
is whether it makes sense for your system. That’s up to you
to decide. The doc page is simply telling you that there are several