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
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
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