Table 1 Potential functions and parameters of united atom force field

$E_{\mathrm{nb}}=\left(\begin{array}{cc}\hfill 4ϵ\left[{\left(\frac{\sigma }{r}\right)}^{12}-{\left(\frac{\sigma }{r}\right)}^6\right]\hfill & \hfill ,r<r_c\hfill \\ \hfill 0\hfill & \hfill ,r\ge r_c\hfill \end{array}\right)$

$E_b=\frac{1}{2}{k}_{\mathrm{b}}{\left(r_b-r_0\right)}^2$

$E_{\mathrm{\theta }}=\frac{1}{2}{k}_{\mathrm{\theta }}{\left(\theta -{\theta }_0\right)}^2$

$E_{\mathrm{\varphi }}={\displaystyle \sum _{n=0}^3{A}_n{cos}^{n-1}}\left(\varphi \right)$