It is possible to treat a virtual atom with fractional nuclear charge by using a pseudopotential with the corresponding fractional nuclear charge. The pseudopotential for the virtual atom can be generated by ADPACK. The relevant keywords in ADPACK are given by
AtomSpecies 6.2 total.electron 6.2 valence.electron 4.2 <occupied.electrons 1 2.0 2 2.0 2.2 occupied.electrons>The above example is for a virtual atom on the way of carbon and nitrogen atoms. Also, it is noted that basis functions for the pseudopotential of the virtual atom must be generated for the virtual atom with the same fractional nuclear charge, since the atomic charge density stored in *.pao is used to make the neutral atom potential.
As an illustration, the DOS of CN calculated using the method is shown in Fig. 26. The input file is 'DIA8-VA.dat' which can be found in the directory, work. In the calculation, one of eight carbon atoms in the unit cell was replaced by a virtual atom with an effective nuclear charge of 4.2, which corresponds to a stoichiometric compound of CN.