forked from lijiext/lammps
120 lines
4.7 KiB
HTML
120 lines
4.7 KiB
HTML
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<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>bond_style quartic command
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</H3>
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<H3>bond_style quartic/omp command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>bond_style quartic
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>bond_style quartic
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bond_coeff 2 1200 -0.55 0.25 1.3 34.6878
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>quartic</I> bond style uses the potential
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</P>
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<CENTER><IMG SRC = "Eqs/bond_quartic.jpg">
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</CENTER>
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<P>to define a bond that can be broken as the simulation proceeds (e.g.
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due to a polymer being stretched). The sigma and epsilon used in the
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LJ portion of the formula are both set equal to 1.0 by LAMMPS.
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</P>
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<P>The following coefficients must be defined for each bond type via the
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<A HREF = "bond_coeff.html">bond_coeff</A> command as in the example above, or in
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the data file or restart files read by the <A HREF = "read_data.html">read_data</A>
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or <A HREF = "read_restart.html">read_restart</A> commands:
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</P>
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<UL><LI>K (energy/distance^4)
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<LI>B1 (distance)
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<LI>B2 (distance)
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<LI>Rc (distance)
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<LI>U0 (energy)
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</UL>
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<P>This potential was constructed to mimic the FENE bond potential for
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coarse-grained polymer chains. When monomers with sigma = epsilon =
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1.0 are used, the following choice of parameters gives a quartic
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potential that looks nearly like the FENE potential: K = 1200, B1 =
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-0.55, B2 = 0.25, Rc = 1.3, and U0 = 34.6878. Different parameters
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can be specified using the <A HREF = "bond_coeff.html">bond_coeff</A> command, but
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you will need to choose them carefully so they form a suitable bond
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potential.
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</P>
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<P>Rc is the cutoff length at which the bond potential goes smoothly to a
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local maximum. If a bond length ever becomes > Rc, LAMMPS "breaks"
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the bond, which means two things. First, the bond potential is turned
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off by setting its type to 0, and is no longer computed. Second, a
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pairwise interaction between the two atoms is turned on, since they
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are no longer bonded.
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</P>
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<P>LAMMPS does the second task via a computational sleight-of-hand. It
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subtracts the pairwise interaction as part of the bond computation.
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When the bond breaks, the subtraction stops. For this to work, the
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pairwise interaction must always be computed by the
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<A HREF = "pair_style.html">pair_style</A> command, whether the bond is broken or
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not. This means that <A HREF = "special_bonds.html">special_bonds</A> must be set
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to 1,1,1, as indicated as a restriction below.
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</P>
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<P>Note that when bonds are dumped to a file via the <A HREF = "dump.html">dump
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local</A> command, bonds with type 0 are not included. The
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<A HREF = "delete_bonds.html">delete_bonds</A> command can also be used to query the
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status of broken bonds or permanently delete them, e.g.:
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</P>
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<PRE>delete_bonds all stats
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delete_bonds all bond 0 remove
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</PRE>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>intel</I>, <I>kk</I>, <I>omp</I>, or <I>opt</I> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This bond style can only be used if LAMMPS was built with the
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MOLECULE package (which it is by default). 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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</P>
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<P>The <I>quartic</I> style requires that <A HREF = "special_bonds.html">special_bonds</A>
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parameters be set to 1,1,1. Three- and four-body interactions (angle,
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dihedral, etc) cannot be used with <I>quartic</I> bonds.
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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 = "bond_coeff.html">bond_coeff</A>, <A HREF = "delete_bonds.html">delete_bonds</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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