The atom type names used under atoms (and in the create record) must begin with the standard chemical element symbol (H, He, Li...). The program uses this to deduce the nuclear charge and other elemental properties.
For the standard elements one can redefine the atomic mass (for instance to define a suitable isotope).
CREATE H {m=value} datafile
value
The atomic mass, which will then override the default value for the indicated chemical element.
A more extensive feature is available to define an artificial chemical element with user-specified properties. Such new elements are denoted Alternative Elements;and may for instance have a non-integer nuclear charge. The chemical symbol of an Alternative Elements is gh (for ghost) or J: either one is ok.
You can create j-type or gh-type basic atoms and use them subsequently as fragments in a molecule.
