g_current

Main Table of Contents

VERSION 4.6.4
Wed 13 Nov 2013

Description

g_current 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 can be used to obtain 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 results from the fit allow the determination of 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 option 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

optionfilenametypedescription
-s topol.tpr Input Structure+mass(db): tpr tpb tpa gro g96 pdb
-n index.ndx Input, Opt. Index file
-f traj.xtc Input Trajectory: xtc trr trj gro g96 pdb cpt
-o current.xvg Output xvgr/xmgr file
-caf caf.xvg Output, Opt. xvgr/xmgr file
-dsp dsp.xvg Output xvgr/xmgr file
-md md.xvg Output xvgr/xmgr file
-mj mj.xvg Output xvgr/xmgr file
-mc mc.xvg Output, Opt. xvgr/xmgr file

Other options

optiontypedefaultdescription
-[no]h bool no Print help info and quit
-[no]version bool no Print version info and quit
-nice int 0 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
-sh int 1000 Shift of the frames for averaging the correlation functions and the mean-square displacement.
-[no]nojump bool yes Removes jumps of atoms across the box.
-eps real 0 Dielectric constant of the surrounding medium. The value zero corresponds to infinity (tin-foil boundary conditions).
-bfit real 100 Begin of the fit of the straight line to the MSD of the translational fraction of the dipole moment.
-efit real 400 End of the fit of the straight line to the MSD of the translational fraction of the dipole moment.
-bvit real 0.5 Begin of the fit of the current autocorrelation function to a*t^b.
-evit real 5 End of the fit of the current autocorrelation function to a*t^b.
-tr real 0.25 Fraction of the trajectory taken into account for the integral.
-temp real 300 Temperature for calculating epsilon.

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