I want to calculate the average stress of TiO2 nanotube during tensile loading. However, the value of volume is a critical value for stress estimation. I am just wondering can I calculate the occupied volume of certain irregular nanostructures? If LAMMPS cannot do this, is there any other method to realize it?

I want to calculate the average stress of TiO2 nanotube during tensile
loading. However, the value of volume is a critical value for stress
estimation. I am just wondering can I calculate the occupied volume of
certain irregular nanostructures? If LAMMPS cannot do this, is there any
other method to realize it?

this is a principal issue. think about it! what is the volume of a single atom?
you can only obtain an "effective volume" for a dense system by using
the total volume. everything else is ill-defined. that being said,
there are multiple ways to approximate this, e.g. through empirically
defining an effective per atom volume for your atoms and then summing
up or by writing a small program that define a suitably fine grid for
the simulation cell and then marks all grid points as "occupied" that
are within a given effective radius from an atom and then count the
number of occupied grid points and multiply is with the volume of a
single voxel. the former can be done with LAMMPS by simply counting
atoms, but is less accurate when atoms overlap a lot. the other would
need to be programmed.