This is an optional top-level directive that allows the user to specify the total charge of the system. The form of the directive is as follows:
CHARGE <real charge default 0>
The charge directive, in conjunction with the charges of atomic nuclei
(which can be changed via the geometry input, cf. Section Geometry),
determines the total number of electrons in the chemical system.
Therefore, a charge n specification removes "n" electrons from the
chemical system. Similarly, charge -n adds "n" electrons. is zero if
this directive is omitted. An example of a case where the directive
would be needed is for a calculation on a doubly charged cation. In such
a case, the directive is simply,
charge 2
If centers with fractional charge have been specified the net charge of the system should be adjusted to ensure that there are an integral number of electrons.
The charge may be changed between tasks, and is used by all wavefunction types. For instance, in order to compute the first two vertical ionization energies of LiH, one might optimize the geometry of LiH using a UHF SCF wavefunction, and then perform energy calculations at the optimized geometry on LiH+ and LiH2+ in turn. This is accomplished with the following input:
geometry; Li 0 0 0; H 0 0 1.64; end basis; Li library 3-21g; H library 3-21g; end
scf; uhf; singlet; end task scf optimize
charge 1 scf; uhf; doublet; end task scf
charge 2 scf; uhf; singlet; end task scf
The GEOMETRY, BASIS, and SCF directives are described below (Geometry, Basis and SCF respectively) but their intent should be clear. The TASK directive is described above (TASK).