The Evaluator class provides an interface to the Chemical Terms library. It requires the creation of a ChemJEP object. ChemJEP is the actual expression parser/compiler and evaluator for the chemical terms in in the Evaluator call. The API for Evaluator presents many fine examples but we will present a simple case as example. This command will calculate the first acidic pKa for a molecule.
For how to get the Molecule from the selected row see the Reading a molecule scriptlet.