.

 where n is the number of pair potentials to be entered. There follows n records, each specifying a particular pair potential in the following manner:

		 atmnam 1 		 a8 		 first atom type


atmnam 2 		 a8 		 second atom type


key 		 a4,1X 		 potential key. See table 4.12


variable 1 		 real 		 potential parameter see table4.12


variable 2 		 real 		 potential parameter see table4.12


variable 3 		 real 		 potential parameter see table4.12


variable 4 		 real 		 potential parameter see table4.12


variable 5 		 real 		 potential parameter see table4.12

The variables pertaining to each potential are described in table 4.12.Note that there is an empty column after the potential key. This means that the potential paramaters are enterd in columns 22 to 81. Note that any pair potential not specified in the FIELD file, will be assumed to be zero. Note also that the Sutton-Chen potential for metals is classified as a pair potential for input purposes.

Table 4.12: Definition of pair potential functions and variables

key	potential type	Variables (1-5)					functional form
12-6	12-6	$A$	$B$				$U(r)=\left (\frac{A}{r^{12}}\right)-\left(\frac{B}{r^{6}}\right)$
lj	Lennard-Jones	$\epsilon$	$\sigma$				$U(r) = 4\epsilon\left[\left(\frac{\sigma}{r}\right)^{12}-\left(\frac{\sigma}{r}\right)^{6}\right ]$
nm	n-m	$E_{o}$	$n$	$m$	$r_{0}$		$U(r)=\frac{E_{o}}{(n-m)}\left[m\left (\frac{r_{o}}{r}\right)^{n}-n\left(\frac{r_{o}}{r}\right)^{m}\right ]$
buck	Buckingham	$A$	$\rho$	$C$			$U(r)=A~\exp\left(-\frac{r}{\rho}\right)-\frac{C}{r^{6}}$
bhm	Born-Huggins	$A$	$B$	$\sigma$	$C$	$D$	$U(r)=A~\exp[B(\sigma-r)]-\frac{C}{r^{6}}-\frac{D}{r^{8}}$
	-Meyer
hbnd	12-10 H-bond	$A$	$B$				$U(r)=\left(\frac{A}{r^{12}}\right)- \left(\frac{B}{r^{10}}\right)$
snm	Shifted force$^{\dagger}$	$E_{o}$	$n$	$m$	$r_{0}$	$r_{c}$$^{\ddagger}$	$U(r)=\frac{\alpha E_{o}}{(n-m)}\times$
	n-m [27]						$\left [m\beta^{n}\left \{ \left (\frac{r_{o}}{r}\right )^{n}- \left(\frac{1}{\g... ...frac{r_{o}}{r}\right )^{m}-\left(\frac{1}{\gamma}\right)^{m}\right \} \right ]$
							$+\frac{nm\alpha E_{o}}{(n-m)} \left ( \frac{r-\gamma r_{o}} {\gamma r_{o}}\righ... ...frac{\beta}{\gamma}\right)^{n} -\left(\frac{\beta}{\gamma}\right )^{m}\right \}$
mors	Morse	$E_{0}$	$r_{0}$	$k$			$U(r)=E_{0}[\{1-\exp(-k(r-r_{0}))\}^{2}-1]$
stch	Sutton-Chen	$\epsilon$	$a$	$n$	$m$	$C$	$U_{i}(r)=\epsilon\left[\frac{1}{2}\sum_{j\ne i}\left(\frac{a}{r_{ij}}\right )^{n} -C\sqrt{\rho_{i}}\right ]$
							$\rho_{i}=\sum_{j\ne i}\left(\frac{a}{r_{ij}}\right )^{m}~~~~~~~$
tab	Tabulation						tabulated potential

$^{\dagger}$ Note: in this formula the terms $\alpha$, $\beta$ and $\gamma$ are compound expressions involving the variables $E_{o},n,m,r_{0}$ and $r_{c}$. See section 2.3.1 for further details.

$^{\ddagger}$ Note: $r_{c}$ defaults to the general van der Waals cutoff (rvdw or rcut) if it is set to zero or not specified or not specified in the FIELD file.

The specification of three body potentials is initiated by the directive: