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+ +
+

fix ehex command

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+

Syntax

+
fix ID group-ID ehex nevery F keyword value
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+
+
    +
  • ID, group-ID are documented in fix command
    • +
  • ehex = style name of this fix command
    • +
  • nevery = add/subtract heat every this many timesteps
    • +
  • F = energy flux into the reservoir (energy/time units)
    • +
  • zero or more keyword/value pairs may be appended to args
    • +
  • keyword = region or constrain or com or hex
    • +
+
+region value = region-ID
+  region-ID = ID of region (reservoir) atoms must be in for added thermostatting force
+constrain value = none
+  apply the constraint algorithm (SHAKE or RATTLE) again at the end of the timestep
+com value = none
+  rescale all sites of a constrained cluster of atom if its COM is in the reservoir
+hex value = none
+  omit the coordinate correction to recover the HEX algorithm
+
+
+
+

Examples

+
# Lennard-Jones, from examples/in.ehex.lj
+
+
+
fix fnve all nve
+# specify regions rhot and rcold
+...
+fix fhot all ehex 1 0.15 region rhot
+fix fcold all ehex 1 -0.15 region rcold
+
+
+
# SPC/E water, from examples/in.ehex.spce
+fix fnve all nve
+# specify regions rhot and rcold
+...
+fix fhot all ehex 1 0.075 region rhot constrain com
+fix fcold all ehex 1 -0.075 region rcold constrain com
+fix frattle all rattle 1e-10 400 0 b 1 a 1
+
+
+
+
+

Description

+

This fix implements the asymmetric version of the enhanced heat +exchange algorithm (Wirnsberger). The eHEX algorithm is +an extension of the heat exchange algorithm (Ikeshoji) and +adds an additional coordinate integration to account for higher-order +truncation terms in the operator splitting. The original HEX +algorithm (implemented as fix heat) is known to +exhibit a slight energy drift limiting the accessible simulation times +to a few nanoseconds. This issue is greatly improved by the new +algorithm decreasing the energy drift by at least a factor of a +hundred (LJ and SPC/E water) with little computational overhead.

+

In both algorithms (non-translational) kinetic energy is constantly +swapped between regions (reservoirs) to impose a heat flux onto the +system. The equations of motion are therefore modified if a particle +i is located inside a reservoir

+Eqs/fix_ehex01.jpg +

where

+Eqs/fix_ehex02.jpg +

We use

+Eqs/fix_ehex03.jpg +

to label those parts of the simulation box which are not +thermostatted.) The input parameter region-ID of this fix +corresponds to k. The energy swap is modelled by introducing an +additional thermostatting force to the equations of motion, such that +the time evolution of coordinates and momenta of particle i becomes +(Wirnsberger)

+Eqs/fix_ehex04.jpg +

The thermostatting force is given by

+Eqs/fix_ehex05.jpg +

where

+Eqs/fix_ehex06.jpg +

is the mass and

+Eqs/fix_ehex07.jpg +

maps the particle position to the respective reservoir. The quantity

+Eqs/fix_ehex08.jpg +

corresponds to the input parameter F, which is the energy flux into +the reservoir. Furthermore,

+Eqs/fix_ehex09.jpg +

and

+Eqs/fix_ehex10.jpg +

denote the non-translational kinetic +energy and the centre of mass velocity of that reservoir. The +thermostatting force does not affect the centre of mass velocities of +the individual reservoirs and the entire simulation box. A derivation +of the equations and details on the numerical implementation with +velocity Verlet in LAMMPS can be found in reference +“(Wirnsberger)”#_Wirnsberger.

+
+

Note

+

This fix only integrates the thermostatting force and must be +combined with another integrator, such as fix nve, to +solve the full equations of motion.

+
+

This fix 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.

+
+

Note

+

If heat is subtracted from the system too aggressively so that +the group’s kinetic energy would go to zero, then LAMMPS will halt +with an error message. Increasing the value of nevery means that +heat is added/subtracted less frequently but in larger portions. The +resulting temperature profile will therefore be the same.

+
+

This fix will default to fix_heat (HEX algorithm) if +the keyword hex is specified.

+
+

Compatibility with SHAKE and RATTLE (rigid molecules):

+

This fix is compatible with fix shake and fix rattle. If either of these constraining algorithms is +specified in the input script and the keyword constrain is set, the +bond distances will be corrected a second time at the end of the +integration step. It is recommended to specify the keyword com in +addition to the keyword constrain. With this option all sites of a +constrained cluster are rescaled, if its centre of mass is located +inside the region. Rescaling all sites of a cluster by the same factor +does not introduce any velocity components along fixed bonds. No +rescaling takes place if the centre of mass lies outside the region.

+
+

Note

+

You can only use the keyword com along with constrain.

+
+

To achieve the highest accuracy it is recommended to use fix rattle with the keywords constrain and com as +shown in the second example. Only if RATTLE is employed, the velocity +constraints will be satisfied.

+
+

Note

+

Even if RATTLE is used and the keywords com and constrain +are both set, the coordinate constraints will not necessarily be +satisfied up to the target precision. The velocity constraints are +satisfied as long as all sites of a cluster are rescaled (keyyword +com) and the cluster does not span adjacent reservoirs. The current +implementation of the eHEX algorithm introduces a small error in the +bond distances, which goes to zero with order three in the +timestep. For example, in a simulation of SPC/E water with a timestep +of 2 fs the maximum relative error in the bond distances was found to +be on the order of :c,image(Eqs/fix_ehex07.jpg) for relatively large +temperature gradients. A higher precision can be achieved by +decreasing the timestep.

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+

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 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

+

This fix is part of the RIGID package. It is only enabled if LAMMPS +was built with that package. See the Making LAMMPS section for more info.

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