# g_current (1) - Linux Manuals

## g_current: calculates dielectric constants for charged systems

## NAME

g_current - calculates dielectric constants for charged systems## SYNOPSIS

**g_current**

**-s**

*topol.tpr*

**-n**

*index.ndx*

**-f**

*traj.xtc*

**-o**

*current.xvg*

**-caf**

*caf.xvg*

**-dsp**

*dsp.xvg*

**-md**

*md.xvg*

**-mj**

*mj.xvg*

**-mc**

*mc.xvg*

**-[no]h**

**-nice**

*int*

**-b**

*time*

**-e**

*time*

**-dt**

*time*

**-[no]w**

**-[no]xvgr**

**-sh**

*int*

**-[no]nojump**

**-eps**

*real*

**-bfit**

*real*

**-efit**

*real*

**-bvit**

*real*

**-evit**

*real*

**-tr**

*real*

**-temp**

*real*

## DESCRIPTION

This is a tool for calculating the current autocorrelation function, the correlation of the rotational and translational dipole moment of the system, and the resulting static dielectric constant. To obtain a reasonable result the index group has to be neutral. Furthermore the routine is capable of extracting the static conductivity from the current autocorrelation function, if velocities are given. Additionally an Einstein-Helfand fit also allows to get the static conductivity.

The flag ** -caf** is for the output of the current autocorrelation function and ** -mc** writes the
correlation of the rotational and translational part of the dipole moment in the corresponding
file. However this option is only available for trajectories containing velocities.Options ** -sh** and ** -tr** are responsible for the averaging and integration of the
autocorrelation functions. Since averaging proceeds by shifting the starting point
through the trajectory, the shift can be modified with ** -sh** to enable the choice of uncorrelated
starting points. Towards the end, statistical inaccuracy grows and integrating the
correlation function only yields reliable values until a certain point, depending on
the number of frames. The option ** -tr** controls the region of the integral taken into account
for calculating the static dielectric constant.

Option ** -temp** sets the temperature required for the computation of the static dielectric constant.

Option ** -eps** controls the dielectric constant of the surrounding medium for simulations using
a Reaction Field or dipole corrections of the Ewald summation (eps=0 corresponds to
tin-foil boundary conditions).

** -[no]nojump** unfolds the coordinates to allow free diffusion. This is required to get a continuous
translational dipole moment, required for the Einstein-Helfand fit. The resuls from the fit allow to
determine the dielectric constant for system of charged molecules. However it is also possible to extract
the dielectric constant from the fluctuations of the total dipole moment in folded coordinates. But this
options has to be used with care, since only very short time spans fulfill the approximation, that the density
of the molecules is approximately constant and the averages are already converged. To be on the safe side,
the dielectric constant should be calculated with the help of the Einstein-Helfand method for
the translational part of the dielectric constant.

## FILES

**-s**

*topol.tpr*

**Input**

**-n*** index.ndx*
**Input, Opt.**

**-f*** traj.xtc*
**Input**

**-o*** current.xvg*
**Output**

**-caf*** caf.xvg*
**Output, Opt.**

**-dsp*** dsp.xvg*
**Output**

**-md*** md.xvg*
**Output**

**-mj*** mj.xvg*
**Output**

**-mc*** mc.xvg*
**Output, Opt.**

## OTHER OPTIONS

**-[no]h**

*no*

**-nice*** int*** 0**

**-b*** time*** 0 **

**-e*** time*** 0 **

**-dt*** time*** 0 **

**-[no]w***no *

**-[no]xvgr***yes *

**-sh*** int*** 1000**

**-[no]nojump***yes *

**-eps*** real*** 0 **

**-bfit*** real*** 100 **

**-efit*** real*** 400 **

**-bvit*** real*** 0.5 **

**-evit*** real*** 5 **

**-tr*** real*** 0.25 **

**-temp*** real*** 300 **