In order to freeze the total charge of a group of atoms (for example, a molecule) one can add a MOLCHARGE key to the geo file in the BGF format. The format of the key is as follows:
MOLCHARGE n1 n2 q
n2 specify indices of the first and the last atoms of the molecule and
q is the charge. All atoms between
n2 will be included in the molecule.
When specifying the MOLCHARGE key all atoms present in the system must be assigned to one of the the molecules. For example, if your system consists of 10 water molecules and
the charge of the first of them is constrained to be +1 and the total charge of the system is zero then the following two MOLCHARGE keys must be specified:
MOLCHARGE 1 3 1.0 MOLCHARGE 4 30 -1.0
Note: The MOLCHARGE is implemented as a simple constraint in the charge equilibration procedure and it does not take any bonding information into account. This means that specifying molecular charges using the MOLCHARGE key makes sense only if you are sure that the constitution of the molecule will not change during calculation. This is useful, for example, when fitting EEM-related parameters in a force-field.