2009-08-11 04:13:44 +08:00
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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 table 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 table style N
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</PRE>
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<UL><LI>style = <I>linear</I> or <I>spline</I> = method of interpolation
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<LI>N = use N values in table
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>angle_style table linear 1000
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angle_coeff 3 file.table ENTRY1
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Style <I>table</I> creates interpolation tables of length <I>N</I> from angle
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2010-11-18 00:26:24 +08:00
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potential and derivative values listed in a file(s) as a function of
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angle The files are read by the <A HREF = "angle_coeff.html">angle_coeff</A>
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command.
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2009-08-11 04:13:44 +08:00
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</P>
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<P>The interpolation tables are created by fitting cubic splines to the
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2010-11-18 00:26:24 +08:00
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file values and interpolating energy and derivative values at each of
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<I>N</I> angles. During a simulation, these tables are used to interpolate
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energy and force values on individual atoms as needed. The
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interpolation is done in one of 2 styles: <I>linear</I> or <I>spline</I>.
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2009-08-11 04:13:44 +08:00
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</P>
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<P>For the <I>linear</I> style, the angle is used to find 2 surrounding table
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values from which an energy or its derivative is computed by linear
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interpolation.
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</P>
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<P>For the <I>spline</I> style, a cubic spline coefficients are computed and
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stored at each of the <I>N</I> values in the table. The angle is used to
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find the appropriate set of coefficients which are used to evaluate a
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cubic polynomial which computes the energy or derivative.
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2009-08-11 04:13:44 +08:00
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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.
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</P>
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<UL><LI>filename
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<LI>keyword
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</UL>
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<P>The filename specifies a file containing tabulated energy and
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derivative values. The keyword specifies a section of the file. The
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format of this file is described below.
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</P>
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<HR>
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<P>The format of a tabulated file is as follows (without the
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parenthesized comments):
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</P>
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<PRE># Angle potential for harmonic (one or more comment or blank lines)
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</PRE>
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<PRE>HAM (keyword is the first text on line)
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N 181 FP 0 0 EQ 90.0 (N, FP, EQ parameters)
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(blank line)
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N 181 FP 0 0 (N, FP parameters)
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1 0.0 200.5 2.5 (index, angle, energy, derivative)
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2 1.0 198.0 2.5
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...
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181 180.0 0.0 0.0
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</PRE>
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<P>A section begins with a non-blank line whose 1st character is not a
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"#"; blank lines or lines starting with "#" can be used as comments
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between sections. The first line begins with a keyword which
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identifies the section. The line can contain additional text, but the
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initial text must match the argument specified in the
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<A HREF = "angle_coeff.html">angle_coeff</A> command. The next line lists (in any
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order) one or more parameters for the table. Each parameter is a
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keyword followed by one or more numeric values.
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</P>
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<P>The parameter "N" is required and its value is the number of table
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entries that follow. Note that this may be different than the <I>N</I>
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specified in the <A HREF = "angle_style.html">angle_style table</A> command. Let
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Ntable = <I>N</I> in the angle_style command, and Nfile = "N" in the
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tabulated file. What LAMMPS does is a preliminary interpolation by
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creating splines using the Nfile tabulated values as nodal points. It
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uses these to interpolate as needed to generate energy and derivative
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values at Ntable different points. The resulting tables of length
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Ntable are then used as described above, when computing energy and
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force for individual angles and their atoms. This means that if you
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want the interpolation tables of length Ntable to match exactly what
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is in the tabulated file (with effectively no preliminary
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interpolation), you should set Ntable = Nfile.
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</P>
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<P>The "FP" parameter is optional. If used, it is followed by two values
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fplo and fphi, which are the 2nd derivatives at the innermost and
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outermost angle settings. These values are needed by the spline
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construction routines. If not specified by the "FP" parameter, they
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are estimated (less accurately) by the first two and last two
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derivative values in the table.
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2009-08-11 04:13:44 +08:00
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</P>
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<P>The "EQ" parameter is also optional. If used, it is followed by a the
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equilibrium angle value, which is used, for example, by the <A HREF = "fix_shake.html">fix
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shake</A> command. If not used, the equilibrium angle is
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set to 180.0.
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</P>
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<P>Following a blank line, the next N lines list the tabulated values.
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On each line, the 1st value is the index from 1 to N, the 2nd value is
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the angle value (in degrees), the 3rd value is the energy (in energy
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units), and the 4th is -dE/d(theta) (also in energy units). The 3rd
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term is the energy of the 3-atom configuration for the specified
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angle. The last term is the derivative of the energy with respect to
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the angle (in degrees, not radians). Thus the units of the last term
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are still energy, not force. The angle values must increase from one
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line to the next. The angle values must also begin with 0.0 and end
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with 180.0, i.e. span the full range of possible angles.
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</P>
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<P>Note that one file can contain many sections, each with a tabulated
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potential. LAMMPS reads the file section by section until it finds
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one that matches the specified keyword.
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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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"molecular" 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><B>Related commands:</B>
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</P>
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<P><A HREF = "angle_coeff.html">angle_coeff</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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