## g_dielectric

Main Table of Contents

VERSION 4.6
Sat 19 Jan 2013

### Description

`g_dielectric` calculates frequency dependent dielectric constants from the autocorrelation function of the total dipole moment in your simulation. This ACF can be generated by `g_dipoles`. The functional forms of the available functions are:

One parameter: y = exp(-a_1 x),
Two parameters: y = a_2 exp(-a_1 x),
Three parameters: y = a_2 exp(-a_1 x) + (1 - a_2) exp(-a_3 x).
Start values for the fit procedure can be given on the command line. It is also possible to fix parameters at their start value, use `-fix` with the number of the parameter you want to fix.

Three output files are generated, the first contains the ACF, an exponential fit to it with 1, 2 or 3 parameters, and the numerical derivative of the combination data/fit. The second file contains the real and imaginary parts of the frequency-dependent dielectric constant, the last gives a plot known as the Cole-Cole plot, in which the imaginary component is plotted as a function of the real component. For a pure exponential relaxation (Debye relaxation) the latter plot should be one half of a circle.

### Files

option | filename | type | description
--- | --- | --- | ---
`-f` | `dipcorr.xvg` | Input | xvgr/xmgr file
`-d` | `deriv.xvg` | Output | xvgr/xmgr file
`-o` | `epsw.xvg` | Output | xvgr/xmgr file
`-c` | `cole.xvg` | Output | xvgr/xmgr file

### Other options

option | type | default | description
--- | --- | --- | ---
`-[no]h` | bool | `no` | Print help info and quit
`-[no]version` | bool | `no` | Print version info and quit
`-nice` | int | `19` | Set the nicelevel
`-b` | time | `0` | First frame (ps) to read from trajectory
`-e` | time | `0` | Last frame (ps) to read from trajectory
`-dt` | time | `0` | Only use frame when t MOD dt = first time (ps)
`-[no]w` | bool | `no` | View output `.xvg`, `.xpm`, `.eps` and `.pdb` files
`-xvg` | enum | `xmgrace` | xvg plot formatting: `xmgrace`, `xmgr` or `none`
`-[no]fft` | bool | `no` | use fast fourier transform for correlation function
`-[no]x1` | bool | `yes` | use first column as <it>x</it>-axis rather than first data set
`-eint` | real | `5` | Time to end the integration of the data and start to use the fit
`-bfit` | real | `5` | Begin time of fit
`-efit` | real | `500` | End time of fit
`-tail` | real | `500` | Length of function including data and tail from fit
`-A` | real | `0.5` | Start value for fit parameter A
`-tau1` | real | `10` | Start value for fit parameter τ1
`-tau2` | real | `1` | Start value for fit parameter τ2
`-eps0` | real | `80` | ε0 of your liquid
`-epsRF` | real | `78.5` | ε of the reaction field used in your simulation. A value of 0 means infinity.
`-fix` | int | `0` | Fix parameters at their start values, A (2), tau1 (1), or tau2 (4)
`-ffn` | enum | `none` | Fit function: `none`, `exp`, `aexp`, `exp_exp`, `vac`, `exp5`, `exp7`, `exp9` or `erffit`
`-nsmooth` | int | `3` | Number of points for smoothing

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