If only one of ∆ -and ∆ + has a non-charge entry, then the charge difference is accounted for in the same dictionary, while the other one is left empty. This is always possible since charges may be positive or negative. Instances of the both-sides case, i.e., instances with missing atoms in both reactants and products are not considered further here. They require a more sophisticated approach and are relegated to the MCS-based method in our current implementation.