* The default value is taken as 0.6 * approximate bond length, according to the reference covalent radii.
However, there are some exceptions:
1) If the pseudo potential is used, you must make sure that the value exceeds the summation of the core radius.
2) if you know that some elements prefer to be very far apart, you can specify this information.
Mg-Mg distance in MgSiO3 can be set much larger (e.g., 2 Å).
Beware, however, that the larger these minimum distances, the more difficult it is to find structures fulfilling these constraints (especially for large systems),
so a compromise must be found in each case.