Table 2.

Coefficients in the expansion of a torsion energy term α $U_{\alpha }\left(\varphi \right)=\sum _n{a}_{n,a}[1+\text{cos}(n·\varphi -{\delta }_{n,a}\left)\right]$. Only the coefficients a_n are optimized. δ_n is fixed at π. Sample of torsion types, their atom types, and their corresponding indices are shown in Figure 5.

Torsion Type		Starting parameters			POP
	a1	a2	a3	a1	a2	a3
43	0.0000	2.5000	0.0000	−0.0001	2.4998	−0.0005
47	0.0000	2.5000	0.0000	0.0000	2.4999	0.0002
45	0.0000	2.5000	0.0000	0.0000	2.4999	0.0004
51	0.0000	2.5000	0.0000	0.0000	2.4999	−0.0003
17	0.0000	0.0000	0.0000	0.0586	−0.1096	0.0305
76	0.0000	0.0000	0.0000	−0.0682	0.0873	0.0434
30	0.0000	0.0000	0.0000	−0.0604	−0.1110	−0.0289
79	0.0000	0.0000	0.0000	0.0704	0.0883	−0.0408
64	0.0000	0.0000	0.1000	0.2167	−0.0354	0.0375
140	0.0000	0.0000	0.0000	−0.2157	0.2981	0.0548
60	0.0000	2.5000	0.0000	0.0000	2.4999	−0.0002
56	0.0000	2.5000	0.0000	0.0000	2.5001	−0.0002
55	0.0000	2.5000	0.0000	0.0000	2.4999	0.0005