The empty atom is used to allocate basis orbitals to a vacant region
such as vacancy and interstitial region. The pseudopotential
for the empty atom is a flat zero potential. One can allocate any basis
functions for the empty atom. How to define the empty atom and allocate
basis function for the empty atom can be found in the manual.
It is also noted that the number of valence electrons is zero.
The empty atom is compatible with all the functionalities of OpenMX.