| ## g_dielectric Main Table of Contents | VERSION 4.0 Sun 18 Jan 2009

### Description

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. For an estimate of the error you can run g_statistics on the ACF, and use the output thus generated for this program. The functional forms of the available functions are:

One parmeter : y = Exp[-a1 x] Two parmeters : y = a2 Exp[-a1 x] Three parmeter: y = a2 Exp[-a1 x] + (1 - a2) Exp[-a3 x] Startvalues for the fit procedure can be given on the commandline. 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
`-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
`-[no]xvgr` | bool | `yes` | Add specific codes (legends etc.) in the output xvg files for the xmgrace program
`-[no]fft` | bool | `no` | use fast fourier transform for correlation function
`-[no]x1` | bool | `yes` | use first column as X axis rather than first data set
`-eint` | real | `5` | Time were 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 tau1
`-tau2` | real | `1` | Start value for fit parameter tau2
`-eps0` | real | `80` | Epsilon 0 of your liquid
`-epsRF` | real | `78.5` | Epsilon 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` or `exp9`
`-nsmooth` | int | `3` | Number of points for smoothing

http://www.gromacs.org
gromacs@gromacs.org