value: atoms in specified group assigned constant value given <br/>
lattice: volume per atom for specified lattice type (e.g. fcc) and parameter <br/>
element: element volume divided among atoms within element <br/>
region: volume per atom determined based on the atom count in the MD regions and their volumes. Note: meaningful only if atoms completely fill all the regions. <br/>
group: volume per atom determined based on the atom count in a group and its volume<br/>
read_in: list of values for atoms are read-in from specified file <br/>