Number of Hydrogen Bond Donors (HBD) 2. The default SMARTS pattern is: [$([O;H1,-&!$(*-N=O)]),$([S;H1&X2,-&X1]),$([#7;H;!$(*(S(=O)=O)C(F)(F)F);!$(n1nnnc1);!$(n1nncn1)]),$([#7;-])].