forked from lijiext/lammps
115 lines
3.7 KiB
Plaintext
115 lines
3.7 KiB
Plaintext
.. index:: boundary
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boundary command
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================
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Syntax
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""""""
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.. parsed-literal::
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boundary x y z
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* x,y,z = *p* or *s* or *f* or *m*, one or two letters
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.. parsed-literal::
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*p* is periodic
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*f* is non-periodic and fixed
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*s* is non-periodic and shrink-wrapped
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*m* is non-periodic and shrink-wrapped with a minimum value
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Examples
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""""""""
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.. parsed-literal::
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boundary p p f
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boundary p fs p
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boundary s f fm
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Description
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"""""""""""
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Set the style of boundaries for the global simulation box in each
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dimension. A single letter assigns the same style to both the lower
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and upper face of the box. Two letters assigns the first style to the
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lower face and the second style to the upper face. The initial size
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of the simulation box is set by the :doc:`read_data <read_data>`,
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:doc:`read_restart <read_restart>`, or :doc:`create_box <create_box>`
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commands.
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The style *p* means the box is periodic, so that particles interact
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across the boundary, and they can exit one end of the box and re-enter
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the other end. A periodic dimension can change in size due to
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constant pressure boundary conditions or box deformation (see the :doc:`fix npt <fix_nh>` and :doc:`fix deform <fix_deform>` commands). The *p*
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style must be applied to both faces of a dimension.
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The styles *f*, *s*, and *m* mean the box is non-periodic, so that
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particles do not interact across the boundary and do not move from one
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side of the box to the other.
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For style *f*, the position of the face is fixed. If an atom moves
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outside the face it will be deleted on the next timestep that
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reneighboring occurs. This will typically generate an error unless
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you have set the :doc:`thermo_modify lost <thermo_modify>` option to
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allow for lost atoms.
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For style *s*, the position of the face is set so as to encompass the
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atoms in that dimension (shrink-wrapping), no matter how far they
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move.
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For style *m*, shrink-wrapping occurs, but is bounded by the value
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specified in the data or restart file or set by the
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:doc:`create_box <create_box>` command. For example, if the upper z
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face has a value of 50.0 in the data file, the face will always be
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positioned at 50.0 or above, even if the maximum z-extent of all the
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atoms becomes less than 50.0. This can be useful if you start a
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simulation with an empty box or if you wish to leave room on one side
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of the box, e.g. for atoms to evaporate from a surface.
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For triclinic (non-orthogonal) simulation boxes, if the 2nd dimension
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of a tilt factor (e.g. y for xy) is periodic, then the periodicity is
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enforced with the tilt factor offset. If the 1st dimension is
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shrink-wrapped, then the shrink wrapping is applied to the tilted box
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face, to encompass the atoms. E.g. for a positive xy tilt, the xlo
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and xhi faces of the box are planes tilting in the +y direction as y
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increases. These tilted planes are shrink-wrapped around the atoms to
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determine the x extent of the box.
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See :ref:`Section_howto 12 <howto_12>` of the doc pages
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for a geometric description of triclinic boxes, as defined by LAMMPS,
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and how to transform these parameters to and from other commonly used
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triclinic representations.
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Restrictions
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""""""""""""
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This command cannot be used after the simulation box is defined by a
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:doc:`read_data <read_data>` or :doc:`create_box <create_box>` command or
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:doc:`read_restart <read_restart>` command. See the
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:doc:`change_box <change_box>` command for how to change the simulation
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box boundaries after it has been defined.
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For 2d simulations, the z dimension must be periodic.
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Related commands
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""""""""""""""""
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See the :doc:`thermo_modify <thermo_modify>` command for a discussion
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of lost atoms.
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Default
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"""""""
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.. parsed-literal::
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boundary p p p
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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