An atom's implicit hydrogen count may or may not correlate with chemical intuition or experimental data. Consider the phosphorous-bearing atom of hypophosphorous acid (HOP(O)H 2 ). We might expect the implicit hydrogen count to equal the experimentally-determined hydrogen count (2). However, the subvalence for the phosphorous atom is found to be three (2 + 1). The implicit hydrogen count is therefore zero (3 -3) rather than the expected two. To represent this atom and other like it, virtual hydrogens must be used.