GAFF

general AMBER force field (GAFF)         http://ambermd.org/antechamber/gaff.pdf
Basic Atom Types in GAFF

Atom type	Description	Atom type	Description
c c1 c2 c3 ca	sp2 C in C=O, C=S sp1 C sp2 C, aliphatic sp3 C sp2 C, aromatic	o oh os	sp2 O in C=O, COO- sp3 O in hydroxyl group sp3 O in ether and ester
n n1 n2 n3 n4 na nh no	sp2 N in amide sp1 N sp2 N with 2 subst. readl double bond sp3 N with 3 subst. sp3 N with 4 subst. sp2 N with 3 subst amine N connected to the aromatic rings N in nitro group	s2 sh ss s4 s6	sp2 S (p=S, C=S etc) sp3 S in thiol group sp3 S in -SR and SS hypervalent S, 3 subst. hypervalent S, 4 subst.
		hc ha hn ho hs hp	H on aliphatic C H on aromatic C H on N H on O H on S H on P
f cl br i	any F any Cl any Br any I	p2 p3 p4 p5	sp2 P (C=P etc) sp3 P, 3 subst. hypervalent P, 3 subst. hypervalent P, 4 subst.

Special Atom Types in GAFF

Atom type	Description	Atom type	Description
h1 h2 h3 h4 h5	H on aliphatic C with 1 EW group; H on aliphatic C with 2 EW group; H on aliphatic C with 3 EW group; H on aromatic C with 4 EW group; H on aromatic C with 5 EW group;	cc(cd) ce(cf) cp(cq) cu cv cx cy	inner sp2 C in conj. ring systems inner sp2 C in conj. chain systems bridge aromatic C sp2 C in three-memberred rings sp2 C in four-memberred rings sp3 C in three-memberred rings sp3 C in four-memberred rings
n nb nc(nd) sx sy	aromatic nitrogen inner sp2 N in conj. ring systems inner sp2 N in conj. chain systems conj. S, 3 subst. conj. S, 4 subst.	pb pc(pd) pe(pf) px py	aromatic phosphorus inner sp2 P in conj. ring systems inner sp2 P in conj. chain systems conj. P, 3 subst. conj. P, 4 subst.

EW: electron-withdraw group