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interactions:dipole

# Dipole Interactions

To enable the calculation of electric dipole moments, potfit has to be compiled with the dipole option. The dipole $\vec P_{i,n}$ of atom $i$ in iteration step $n$ is then calculated self-consistently by the use of the Tangney-Scandolo potential model1).

$$\vec P_{i,n} = \vec P_{i,\text{NF}} + \vec P_{i,\text{IND}}$$

The near field (NF) part,

$$\vec P_{i,\text{NF}} = \alpha \sum\limits_{j \neq i} \frac{q_j \vec r_{ij}}{r_{ij}^3} f_{ij}$$

is caused by the electric field of nearby charges. The induced (IND) part,

$$\vec P_{i,\text{IND}} = \alpha \vec E (\vec P_{j,n-1}),$$

is due to the electric field of the other dipole moments. $\alpha$ is the polarizability of the considered atom type and $f_{ij}$ is an ad hoc introduced function to account for multipole effects of nearest neighbors.

dipole implies option coulomb, because charges are needed to evaluate the dipole moments.

## Parameters

dipole can be used without specifying additional parameters in the parameter file, because everything works with default values. However, advanced users can specify two new parameters:

dp_tol - float - 1.e-7
dipole iteration precision.

dp_mix - float - 0.2
mixing parameter for dipole convergence during iteration.

## Compatibilities

• dipole has to be compiled with option apot.
• dipole implies coulomb.
• dipole can be used with stress, fweight, evo and can also be executed in parallel using option mpi.
• dipole can not be used together with other force-field approaches (pair, adp, eam, …).

## Potential file

When using dipole, the following parameters have to given in the potential file, straight after the charges:

• alpha polarisability for each atom type.
• b and c parameters of the short-range dipole-model, have to be given for each interaction.

Example for the diatomic oxide SiO2 (contains coulomb-parameters):

#F 0 3
#C Si O
#I 0 0 0
#E

elstat
ratio       1 2
charge_Si   value min max
kappa       value min max
alpha_Si    value min max
alpha_O     value min max
b_SiSi      value min max
b_SiO       value min max
b_OO        value min max
c_SiSi      value min max
c_SiO       value min max
c_O         value min max

## Number of potential functions

To describe a system with $N$ atom types you need $N(N+1)/2$ potentials.

$N$ $N(N+1)/2$
1 1
2 3
3 6

## 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 pair potentials in the potential file for $N$ atom types is:

$\Phi_{00}, \ldots, \Phi_{0N}, \Phi_{11}, \ldots, \Phi_{1N}, \ldots, \Phi_{NN}$

1)
P. Tangney and S. Scandolo, J. Chem. Phys. 117, 8898 (2002) 