forked from lijiext/lammps
127 lines
5.0 KiB
HTML
127 lines
5.0 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>dihedral_style command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>dihedral_style style
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</PRE>
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<UL><LI>style = <I>none</I> or <I>hybrid</I> or <I>charmm</I> or <I>class2</I> or <I>harmonic</I> or <I>helix</I> or <I>multi/harmonic</I> or <I>opls</I>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>dihedral_style harmonic
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dihedral_style multi/harmonic
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dihedral_style hybrid harmonic charmm
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Set the formula(s) LAMMPS uses to compute dihedral interactions
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between quadruplets of atoms, which remain in force for the duration
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of the simulation. The list of dihedral quadruplets is read in by a
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A> command
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from a data or restart file.
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</P>
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<P>Hybrid models where dihedrals are computed using different dihedral
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potentials can be setup using the <I>hybrid</I> dihedral style.
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</P>
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<P>The coefficients associated with a dihedral style can be specified in
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a data or restart file or via the <A HREF = "dihedral_coeff.html">dihedral_coeff</A>
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command.
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</P>
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<P>All dihedral potentials store their coefficient data in binary restart
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files which means dihedral_style and
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<A HREF = "dihedral_coeff.html">dihedral_coeff</A> commands do not need to be
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re-specified in an input script that restarts a simulation. See the
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<A HREF = "read_restart.html">read_restart</A> command for details on how to do
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this. The one exception is that dihedral_style <I>hybrid</I> only stores
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the list of sub-styles in the restart file; dihedral coefficients need
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to be re-specified.
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</P>
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<P>IMPORTANT NOTE: When both a dihedral and pair style is defined, the
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<A HREF = "special_bonds.html">special_bonds</A> command often needs to be used to
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turn off (or weight) the pairwise interaction that would otherwise
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exist between 4 bonded atoms.
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</P>
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<P>In the formulas listed for each dihedral style, <I>phi</I> is the torsional
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angle defined by the quadruplet of atoms. This angle has a sign
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convention as shown in this diagram:
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</P>
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<CENTER><IMG SRC = "Eqs/dihedral_sign.jpg">
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</CENTER>
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<P>where the I,J,K,L ordering of the 4 atoms that define the dihedral
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is from left to right.
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</P>
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<P>This sign convention effects several of the dihedral styles listed
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below (e.g. charmm, helix) in the sense that the energy formula
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depends on the sign of phi, which may be reflected in the value of the
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coefficients you specify.
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</P>
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<P>Here are other important points to take note of when defining the
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LAMMPS dihedral coefficients in the formulas for varoius styles, so
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that they are compatible with other force fields:
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</P>
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<UL><LI>The LAMMPS convention is that the trans position = 180 degrees, while
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in some force fields trans = 0 degrees.
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<LI>Some force fields divide/multiply the prefactor <I>K</I> by the number of
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multiple torsions that contain the j-k bond in an i-j-k-l torsion.
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<LI>Some force fields reverse the sign convention on <I>d</I>.
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<LI>Some force fields let <I>n</I> be positive or negative which corresponds to
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<I>d</I> = 1 or -1 for the harmonic style.
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</UL>
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<HR>
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<P>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 <A HREF = "dihedral_coeff.html">dihedral_coeff</A> command.
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</P>
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<P>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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<A HREF = "Section_commands.html#cmd_5">this page</A>.
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</P>
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<UL><LI><A HREF = "dihedral_none.html">dihedral_style none</A> - turn off dihedral interactions
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<LI><A HREF = "dihedral_hybrid.html">dihedral_style hybrid</A> - define multiple styles of dihedral interactions
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</UL>
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<UL><LI><A HREF = "dihedral_charmm.html">dihedral_style charmm</A> - CHARMM dihedral
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<LI><A HREF = "dihedral_class2.html">dihedral_style class2</A> - COMPASS (class 2) dihedral
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<LI><A HREF = "dihedral_harmonic.html">dihedral_style harmonic</A> - harmonic dihedral
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<LI><A HREF = "dihedral_helix.html">dihedral_style helix</A> - helix dihedral
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<LI><A HREF = "dihedral_multi_harmonic.html">dihedral_style multi/harmonic</A> - multi-harmonic dihedral
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<LI><A HREF = "dihedral_opls.html">dihedral_style opls</A> - OPLS dihedral
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</UL>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>Dihedral styles can only be set for atom styles that allow dihedrals
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to be defined.
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</P>
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<P>Most dihedral styles are part of the MOLECULAR package. They are only
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enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info on packages.
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The doc pages for individual dihedral potentials tell if it is part of
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a package.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "dihedral_coeff.html">dihedral_coeff</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>dihedral_style none
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</P>
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</HTML>
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