Syntax:
fix ID group-ID heat N eflux
Examples:
fix 3 qin heat 1 1.0 fix 4 qout heat 1 -1.0
Description:
Add non-translational kinetic energy (heat) to a group of atoms such that their aggregate momentum is conserved. Two of these fixes can be used to establish a temperature gradient across a simulation domain by adding heat to one group of atoms (hot reservoir) and subtracting heat from another (cold reservoir). E.g. a simulation sampling from the McDLT ensemble. Note that the fix is applied to a group of atoms, not a geometric region, so that the same set of atoms is affected wherever they may move to.
Heat addition/subtraction is performed every N timesteps. The eflux parameter determines the change in aggregate energy of the entire group of atoms. Since eflux is in units of energy/time, this means a larger value of N will add/subtract a larger amount of energy each timestep the fix is invoked. If heat is subtracted from the system too aggressively so that the group's kinetic energy goes to zero, LAMMPS halts with an error message.
Fix heat is different from a thermostat such as fix nvt or fix temp/rescale in that energy is added/subtracted continually. Thus if there isn't another mechanism in place to counterbalance this effect, the entire system will heat or cool continuously. You can use multiple heat fixes so that the net energy change is 0.0 or use fix viscous to drain energy from the system.
This fix does not change the coordinates of its atoms; it only scales their velocities. Thus you must still use an integration fix (e.g. fix nve) on the affected atoms. This fix should not normally be used on atoms that have their temperature controlled by another fix - e.g. fix nvt or fix langevin fix.
Restart, fix_modify, output, run start/stop, minimize info:
No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global scalar or vector or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.
Restrictions: none
Related commands:
compute temp, compute temp/region
Default: none