AtomType NDonor [N&!H0&v3,N&!H0&+1&v4,n&H1&+0]
AtomType ChalcDonor [O,S;H1;+0]
DefineFeature SingleAtomDonor [{NDonor},{ChalcDonor},!$([D1]-[C;D3]=[O,S,N])]
  Family Donor
  Weights 1
EndFeature

AtomType NAcceptor [$([N&v3;H1,H2]-[!$(*=[O,N,P,S])])]
Atomtype NAcceptor [$([N;v3;H0])]
AtomType NAcceptor [$([n;+0])]
AtomType ChalcAcceptor [$([O,S;H1;v2]-[!$(*=[O,N,P,S])])]
AtomType ChalcAcceptor [O,S;H0;v2]
Atomtype ChalcAcceptor [O,S;-]
Atomtype ChalcAcceptor [o,s;+0]
AtomType HalogenAcceptor [F]
DefineFeature SingleAtomAcceptor [{NAcceptor},{ChalcAcceptor},{HalogenAcceptor}]
  Family Acceptor
  Weights 1
EndFeature

# this one is delightfully easy:
DefineFeature AcidicGroup [C,S](=[O,S,P])-[O;H1,H0&-1]
  Family NegIonizable
  Weights 1.0,1.0,1.0
EndFeature

AtomType CarbonOrArom_NonCarbonyl [$([C,a]);!$([C,a](=O))]
AtomType BasicNH2 [$([N;H2&+0][{CarbonOrArom_NonCarbonyl}])]
AtomType BasicNH1 [$([N;H1&+0]([{CarbonOrArom_NonCarbonyl}])[{CarbonOrArom_NonCarbonyl}])]
AtomType BasicNH0 [$([N;H0&+0]([{CarbonOrArom_NonCarbonyl}])([{CarbonOrArom_NonCarbonyl}])[{CarbonOrArom_NonCarbonyl}])]
AtomType BasicNakedN [N,n;X2;+0]
DefineFeature BasicGroup [{BasicNH2},{BasicNH1},{BasicNH0},{BasicNakedN}]
  Family PosIonizable
  Weights 1.0
EndFeature

# aromatic rings of various sizes:
DefineFeature Arom5 a1aaaa1
  Family Aromatic
  Weights 1.0,1.0,1.0,1.0,1.0
EndFeature
DefineFeature Arom6 a1aaaaa1
  Family Aromatic
  Weights 1.0,1.0,1.0,1.0,1.0,1.0
EndFeature
DefineFeature Arom7 a1aaaaaa1
  Family Aromatic
  Weights 1.0,1.0,1.0,1.0,1.0,1.0,1.0
EndFeature
DefineFeature Arom8 a1aaaaaaa1
  Family Aromatic
  Weights 1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0
EndFeature