---
name: "g_dielectric"
section: "1"
platform: "linux"
package: "gromacs-data"
version: "5.0.2-1"
release: "Debian 8 (Jessie)"
platforms: ["linux"]
url: "https://new.carta.tech/gmx-dielectric"
---

# g\_dielectric(1)

Calculate frequency dependent dielectric constants

Part of [gmx-dielectric(1)](/gmx-dielectric/1.md).

## NAME

gmx-dielectric - Calculate frequency dependent dielectric constants

## SYNOPSIS

gmx dielectric \[-f \[\<.xvg\>\]\] \[-d \[\<.xvg\>\]\] \[-o \[\<.xvg\>\]\]\
              \[-c \[\<.xvg\>\]\] \[-nice \<int\>\] \[-b \<time\>\] \[-e \<time\>\]\
              \[-dt \<time\>\] \[-\[no\]w\] \[-xvg \<enum\>\] \[-\[no\]fft\] \[-\[no\]x1\]\
              \[-eint \<real\>\] \[-bfit \<real\>\] \[-efit \<real\>\]\
              \[-tail \<real\>\] \[-A \<real\>\] \[-tau1 \<real\>\] \[-tau2 \<real\>\]\
              \[-eps0 \<real\>\] \[-epsRF \<real\>\] \[-fix \<int\>\]\
              \[-ffn \<enum\>\] \[-nsmooth \<int\>\]

## DESCRIPTION

**gmx dielectric** calculates frequency dependent dielectric constants from the autocorrelation function of the total dipole moment in your simulation. This ACF can be generated by **gmx 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.

## OPTIONS

Options to specify input and output files:

**-f** *\[\<.xvg\>\] (dipcorr.xvg) (Input)*\
     xvgr/xmgr file

**-d** *\[\<.xvg\>\] (deriv.xvg) (Output)*\
     xvgr/xmgr file

**-o** *\[\<.xvg\>\] (epsw.xvg) (Output)*\
     xvgr/xmgr file

**-c** *\[\<.xvg\>\] (cole.xvg) (Output)*\
     xvgr/xmgr file

Other options:

**-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**  *(no)*\
     View output **.xvg**, **.xpm**, **.eps** and **.pdb** files

**-xvg** *\<enum\> (xmgrace)*\
     xvg plot formatting: xmgrace, xmgr, none

**-\[no\]fft**  *(no)*\
     use fast fourier transform for correlation function

**-\[no\]x1**  *(yes)*\
     use first column as *x*-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 tau1

**-tau2** *\<real\> (1)*\
     Start value for fit parameter tau2

**-eps0** *\<real\> (80)*\
     epsilon0 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, exp9, erffit

**-nsmooth** *\<int\> (3)*\
     Number of points for smoothing

## SEE ALSO

**[gromacs(7)](/gromacs/7.md)**

More information about **GROMACS** is available at \<*http://www.gromacs.org/*\>.