diff --git a/doc/src/fix_press_langevin.rst b/doc/src/fix_press_langevin.rst index cb46d2b7d9..9401e035a3 100644 --- a/doc/src/fix_press_langevin.rst +++ b/doc/src/fix_press_langevin.rst @@ -16,15 +16,15 @@ Syntax .. parsed-literal:: one or more keyword value pairs may be appended - keyword = *iso* or *aniso* or *x* or *y* or *z* or *couple* or *dilate* or *modulus* or *temp* - *iso* or *aniso* values = Pstart Pstop Pdamp + keyword = *iso* or *aniso* or *tri* *x* or *y* or *z* or *xy* or *xz* or *yz* or *couple* or *dilate* or *modulus* or *temp* + *iso* or *aniso* or *tri* values = Pstart Pstop Pdamp Pstart,Pstop = scalar external pressure at start/end of run (pressure units) Pdamp = pressure damping parameter - *x* or *y* or *z* values = Pstart Pstop Pdamp + *x* or *y* or *z* or *xy* or *xz* or *yz* values = Pstart Pstop Pdamp Pstart,Pstop = external stress tensor component at start/end of run (pressure units) Pdamp = stress damping parameter *couple* = *none* or *xyz* or *xy* or *yz* or *xz* - *friction* value = Friction to apply to the barostat + *friction* value = Alpha friction value to apply to the barostat *temp* values = Tstart, Tstop Tstart, Tstop = target temperature used for the barostat at start/end of run *dilate* value = *all* or *partial* @@ -95,26 +95,23 @@ discussion of different ways to perform barostatting. ---------- -The barostat is specified using one or more of the *iso*, *aniso*, -*x*, *y*, *z*, and *couple* keywords. These keywords give you the -ability to specify the 3 diagonal components of an external stress -tensor, and to couple various of these components together so that the -dimensions they represent are varied together during a -constant-pressure simulation. Unlike the :doc:`fix npt ` and -:doc:`fix nph ` commands, this fix cannot be used with triclinic -(non-orthogonal) simulation boxes to control all 6 components of the -general pressure tensor. +The barostat is specified using one or more of the *iso*, *aniso*, *tri* *x*, +*y*, *z*, *xy*, *xz*, *yz*, and *couple* keywords. These keywords give you the +ability to specify the 3 diagonal components of an external stress tensor, and +to couple various of these components together so that the dimensions they +represent are varied together during a constant-pressure simulation. -The target pressures for each of the 3 diagonal components of the -stress tensor can be specified independently via the *x*, *y*, *z*, -keywords, which correspond to the 3 simulation box dimensions. For -each component, the external pressure or tensor component at each -timestep is a ramped value during the run from *Pstart* to *Pstop*\ . -If a target pressure is specified for a component, then the -corresponding box dimension will change during a simulation. For -example, if the *y* keyword is used, the y-box length will change. A -box dimension will not change if that component is not specified, -although you have the option to change that dimension via the :doc:`fix deform ` command. +The target pressures for each of the 6 diagonal components of the stress tensor +can be specified independently via the *x*, *y*, *z*, keywords, which +correspond to the 3 simulation box dimensions, and the *xy*, *xz and *yz* +keywords which corresponds to the 3 simulqtion box tilt factors. For each +component, the external pressure or tensor component at each timestep is a +ramped value during the run from *Pstart* to *Pstop*\ . If a target pressure is +specified for a component, then the corresponding box dimension will change +during a simulation. For example, if the *y* keyword is used, the y-box length +will change. A box dimension will not change if that component is not +specified, although you have the option to change that dimension via the +:doc:`fix deform ` command. For all barostat keywords, the *Pdamp* parameter determines the "mass" of the pseudo particle acting as a barostat. The relation is such that :math:`P_{damp} @@ -137,7 +134,7 @@ dimension is simply ignored. ---------- -The *iso* and *aniso* keywords are simply shortcuts that are +The *iso*, *aniso* and *tri* keywords are simply shortcuts that are equivalent to specifying several other keywords together. The keyword *iso* means couple all 3 diagonal components together when @@ -165,6 +162,24 @@ these 4 keywords: z Pstart Pstop Pdamp couple none +The keyword *tri* is the same as *aniso* but also adds the control on the +shear pressure coupled with the tilt factors. + +.. parsed-literal:: + + x Pstart Pstop Pdamp + y Pstart Pstop Pdamp + z Pstart Pstop Pdamp + xy Pstart Pstop Pdamp + xz Pstart Pstop Pdamp + yz Pstart Pstop Pdamp + couple none + +---------- + +The *friction* keyword sets the friction parameter :math:`\alpha` in the +equations of movement of the barostat. All the barostat use the same value. + ---------- This fix computes pressure each timestep. To do @@ -173,7 +188,10 @@ as if this command had been issued: .. code-block:: LAMMPS - compute fix-ID_press group-ID pressure fix-ID_NULL virial + compute fix-ID_press group-ID pressure NULL virial + +The kinetic contribution to the pressure is taken as the ensemble value +:math:`\frac{Nk_bT}{V}` and computed by the fix itself. See the :doc:`compute pressure ` command for details. Note that the IDs of the new compute is the fix-ID + underscore + "press" and the @@ -200,9 +218,12 @@ in its pressure calculations. No global or per-atom quantities are stored by this fix for access by various :doc:`output commands `. -This fix can ramp its target pressure over multiple runs, using the -*start* and *stop* keywords of the :doc:`run ` command. See the -:doc:`run ` command for details of how to do this. +This fix can ramp its target pressure and temperature over multiple runs, using +the *start* and *stop* keywords of the :doc:`run ` command. See the +:doc:`run ` command for details of how to do this. It is recommended that +the ramped temperature is the same as the effective temperature of the +thermalised system. That is, if the system's temperature is ramped by other +commands, it is recommended to do the same with this pressure control. This fix is not invoked during :doc:`energy minimization `.