forked from lijiext/lammps
131 lines
4.2 KiB
Plaintext
131 lines
4.2 KiB
Plaintext
.. index:: dihedral_coeff
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dihedral_coeff command
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======================
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Syntax
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""""""
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.. parsed-literal::
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dihedral_coeff N args
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* N = dihedral type (see asterisk form below)
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* args = coefficients for one or more dihedral types
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Examples
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""""""""
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.. parsed-literal::
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dihedral_coeff 1 80.0 1 3
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dihedral_coeff * 80.0 1 3 0.5
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dihedral_coeff 2* 80.0 1 3 0.5
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Description
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"""""""""""
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----------
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Specify the dihedral force field coefficients for one or more dihedral types.
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The number and meaning of the coefficients depends on the dihedral style.
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Dihedral coefficients can also be set in the data file read by the
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:doc:`read_data <read_data>` command or in a restart file.
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N can be specified in one of two ways. An explicit numeric value can
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be used, as in the 1st example above. Or a wild-card asterisk can be
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used to set the coefficients for multiple dihedral types. This takes the
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form "*" or "*n" or "n*" or "m*n". If N = the number of dihedral types,
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then an asterisk with no numeric values means all types from 1 to N. A
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leading asterisk means all types from 1 to n (inclusive). A trailing
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asterisk means all types from n to N (inclusive). A middle asterisk
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means all types from m to n (inclusive).
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Note that using a dihedral_coeff command can override a previous setting
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for the same dihedral type. For example, these commands set the coeffs
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for all dihedral types, then overwrite the coeffs for just dihedral type 2:
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.. parsed-literal::
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dihedral_coeff * 80.0 1 3
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dihedral_coeff 2 200.0 1 3
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A line in a data file that specifies dihedral coefficients uses the exact
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same format as the arguments of the dihedral_coeff command in an input
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script, except that wild-card asterisks should not be used since
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coefficients for all N types must be listed in the file. For example,
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under the "Dihedral Coeffs" section of a data file, the line that
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corresponds to the 1st example above would be listed as
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.. parsed-literal::
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1 80.0 1 3
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The :doc:`dihedral_style class2 <dihedral_class2>` is an exception to
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this rule, in that an additional argument is used in the input script
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to allow specification of the cross-term coefficients. See its doc
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page for details.
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.. note::
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When comparing the formulas and coefficients for various LAMMPS
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dihedral styles with dihedral equations defined by other force fields,
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note that some force field implementations divide/multiply the energy
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prefactor *K* by the multiple number of torsions that contain the J-K
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bond in an I-J-K-L torsion. LAMMPS does not do this, i.e. the listed
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dihedral equation applies to each individual dihedral. Thus you need
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to define *K* appropriately to account for this difference if
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necessary.
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----------
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Here is an alphabetic list of dihedral styles defined in LAMMPS. Click on
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the style to display the formula it computes and coefficients
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specified by the associated :doc:`dihedral_coeff <dihedral_coeff>` command.
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Note that there are also additional dihedral styles submitted by users
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which are included in the LAMMPS distribution. The list of these with
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links to the individual styles are given in the dihedral section of
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:ref:`this page <cmd_5>`.
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* :doc:`dihedral_style none <dihedral_none>` - turn off dihedral interactions
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* :doc:`dihedral_style hybrid <dihedral_hybrid>` - define multiple styles of dihedral interactions
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* :doc:`dihedral_style charmm <dihedral_charmm>` - CHARMM dihedral
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* :doc:`dihedral_style class2 <dihedral_class2>` - COMPASS (class 2) dihedral
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* :doc:`dihedral_style harmonic <dihedral_harmonic>` - harmonic dihedral
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* :doc:`dihedral_style helix <dihedral_helix>` - helix dihedral
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* :doc:`dihedral_style multi/harmonic <dihedral_multi_harmonic>` - multi-harmonic dihedral
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* :doc:`dihedral_style opls <dihedral_opls>` - OPLS dihedral
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----------
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Restrictions
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""""""""""""
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This command must come after the simulation box is defined by a
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:doc:`read_data <read_data>`, :doc:`read_restart <read_restart>`, or
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:doc:`create_box <create_box>` command.
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A dihedral style must be defined before any dihedral coefficients are
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set, either in the input script or in a data file.
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Related commands
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""""""""""""""""
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:doc:`dihedral_style <dihedral_style>`
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**Default:** none
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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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