Given an atom whose element has one or more default valences, subvalence can be computed (Algorithm 1). Subvalence is the number of hydrogens that can be added to an atom without exceeding the lowest possible default valence. If no suitable default valence exists, then subvalence equals zero. The algorithm begins by computing the atom's valence. Next, the ordered list of default va- For a representative subvalence computation, consider an isolated nitrogen atom without virtual hydrogens. The atom's bond order sum is zero. Its default valences are 3 and 5. The difference between the atom's bond order sum and the first default valence is found to be three (3 -0). Therefore, the subvalence of this atom is three.