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interactions:angular_pair_potentials

# Angular Pair Potentials

The angular pair potentials are a combination of the regular pair potentials and a angular dependent term similar to MEAM:

$$V=\frac{1}{2}\sum\phi_{ij}(r_{ij})+\frac{1}{2}\sum f_{ij}(r_{ij})f_{ik}(r_{ik})g_i(\cos(\theta_{ijk}))$$

With this approach it is possible to fit potentials such as this one here.

Or it can be used to consider non-bonded angular interactions as DL_POLY does for the three-body terms (see 'tbp' interaction, p. 174 in the manual). This can be achieved defining the $f_{ij} = 1$ between the pairs that will form 'triple-bonds' and with their cutoff controlling the central radius to account for angular bonded neighbours. Then, just choosing the $g$ function as an harmonic one.

## Number of potential functions

To describe a system of $N$ atom types you need $N(N+2)$ potentials.

# atom types $\phi_{ij}$ $f_{ij}$ $g_i$ Total # potentials
$N$ $N(N+1)/2$ $N(N+1)/2$ $N$ $N(N+2)$
1 1 1 1 3
2 3 3 2 8
3 6 6 3 15
4 10 10 4 24

## Order of potential functions

The potential table is assumed to be symmetric, i.e. the potential for the atom types 1-0 is the same as the potential 0-1.

The order of the potentials in the potential file for $N$ atom types is:

$\phi_{00}, \ldots, \phi_{0N}, \phi_{11}, \ldots, \phi_{1N}, \ldots, \phi_{NN}$
$f_{00}, \ldots, f_{0N}, f_{11}, \ldots, f_{1N}, \ldots, f_{NN}$
$g_0, \ldots, g_N$ 