~~NOTOC~~
====== 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 model((P. Tangney and S. Scandolo, //J. Chem. Phys.// **117**, 8898 (2002) )).
$$\vec P_{i,n} = \vec P_{i,\text{NF}} + \vec P_{i,\text{IND}}$$
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|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|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
An entire potential file can be downloaded here: [[examples:potentials|Examples]]
===== 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:
$\Phi_{00}, \ldots, \Phi_{0N}, \Phi_{11}, \ldots, \Phi_{1N}, \ldots, \Phi_{NN}$