Table 3.

Atomic charges and polarizability for TMA calculated using the original and refined CHEQ force fields compared to ab initio computed MP2/cc-pVTZ values

Atomic charge	CHARMM (fixed-charge)	CHEQ (original)	CHEQ (refined)	MP2/cc-pVTZ
N	−0.600	0.194	0.185	0.187
C1	−0.350	−0.284	−0.294	−0.271
C2	−0.350	−0.284	−0.294	−0.294
C3	−0.350	−0.284	−0.294	−0.291
C4	−0.350	−0.284	−0.294	−0.279
H11	0.250	0.162	0.166	0.159
H12	0.250	0.162	0.166	0.159
H13	0.250	0.162	0.166	0.159
H21	0.250	0.162	0.166	0.165
H22	0.250	0.162	0.166	0.165
H23	0.250	0.162	0.166	0.165
H31	0.250	0.162	0.166	0.164
H32	0.250	0.162	0.166	0.164
H33	0.250	0.162	0.166	0.164
H41	0.250	0.162	0.166	0.162
H42	0.250	0.162	0.166	0.161
H43	0.250	0.162	0.166	0.161
Polarizability (Å3)	n/a	10.05	7.53	7.58

Ab initio values were computed at the MP2/cc-pVTZ level of theory using Gaussian 03. The fixed charges used in the nonpolarizable CHARMM force field are also shown.