forked from lijiext/lammps
130 lines
4.7 KiB
HTML
130 lines
4.7 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>angle_style dipole command
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</H3>
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<H3>angle_style dipole/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>angle_style dipole
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>angle_style dipole
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angle_coeff 6 2.1 180.0
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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>dipole</I> angle style is used to control the orientation of a dipolar
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atom within a molecule <A HREF = "#Orsi">(Orsi)</A>. Specifically, the <I>dipole</I> angle
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style restrains the orientation of a point dipole mu_j (embedded in atom
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'j') with respect to a reference (bond) vector r_ij = r_i - r_j, where 'i'
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is another atom of the same molecule (typically, 'i' and 'j' are also
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covalently bonded).
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</P>
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<P>It is convenient to define an angle gamma between the 'free' vector mu_j
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and the reference (bond) vector r_ij:
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</P>
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<CENTER><IMG SRC = "Eqs/angle_dipole_gamma.jpg">
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</CENTER>
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<P>The <I>dipole</I> angle style uses the potential:
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</P>
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<CENTER><IMG SRC = "Eqs/angle_dipole_potential.jpg">
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</CENTER>
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<P>where K is a rigidity constant and gamma0 is an equilibrium (reference)
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angle.
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</P>
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<P>The torque on the dipole can be obtained by differentiating the
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potential using the 'chain rule' as in appendix C.3 of
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<A HREF = "#Allen">(Allen)</A>:
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</P>
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<CENTER><IMG SRC = "Eqs/angle_dipole_torque.jpg">
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</CENTER>
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<P>Example: if gamma0 is set to 0 degrees, the torque generated by
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the potential will tend to align the dipole along the reference
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direction defined by the (bond) vector r_ij (in other words, mu_j is
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restrained to point towards atom 'i').
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</P>
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<P>Note that the angle dipole potential does not give rise to any force,
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because it does not depend on the distance between i and j (it only
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depends on the angle between mu_j and r_ij).
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</P>
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<P>The following coefficients must be defined for each angle type via the
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<A HREF = "angle_coeff.html">angle_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)
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<LI>gamma0 (degrees)
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</UL>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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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_6">-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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<P><B>Restrictions:</B>
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</P>
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<P>This angle style can only be used if LAMMPS was built with the
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USER-MISC package. See the <A HREF = "Section_start.html#2_3">Making LAMMPS</A>
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section for more info on packages.
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</P>
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<P>IMPORTANT NOTE: In the "Angles" section of the data file, the atom ID
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'j' corresponding to the dipole to restrain must come before the atom
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ID of the reference atom 'i'. A third atom ID 'k' must also be
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provided, although 'k' is just a 'dummy' atom which can be any atom;
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it may be useful to choose a convention (e.g., 'k'='i') and adhere to
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it. For example, if ID=1 for the dipolar atom to restrain, and ID=2
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for the reference atom, the corresponding line in the "Angles" section
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of the data file would read: X X 1 2 2
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</P>
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<P>The "newton" command for intramolecular interactions must be "on"
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(which is the default).
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</P>
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<P>This angle style should not be used with SHAKE.
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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 = "angle_coeff.html">angle_coeff</A>, <A HREF = "angle_hybrid.html">angle_hybrid</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "Orsi"></A>
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<P><B>(Orsi)</B> Orsi & Essex, The ELBA force field for coarse-grain modeling of
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lipid membranes, PloS ONE 6(12): e28637, 2011.
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</P>
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<A NAME = "Allen"></A>
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<P><B>(Allen)</B> Allen & Tildesley, Computer Simulation of Liquids,
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Clarendon Press, Oxford, 1987.
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</P>
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</HTML>
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