160 lines
5.8 KiB
ReStructuredText
160 lines
5.8 KiB
ReStructuredText
.. index:: angle_style table
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.. index:: angle_style table/omp
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angle_style table command
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=========================
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Accelerator Variants: *table/omp*
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Syntax
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""""""
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.. code-block:: LAMMPS
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angle_style table style N
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* style = *linear* or *spline* = method of interpolation
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* N = use N values in table
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Examples
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""""""""
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.. code-block:: LAMMPS
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angle_style table linear 1000
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angle_coeff 3 file.table ENTRY1
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Description
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"""""""""""
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Style *table* creates interpolation tables of length *N* from angle
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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 :doc:`angle_coeff <angle_coeff>`
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command.
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The interpolation tables are created by fitting cubic splines to the
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file values and interpolating energy and derivative values at each of
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*N* 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: *linear* or *spline*\ .
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For the *linear* 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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For the *spline* style, a cubic spline coefficients are computed and
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stored at each of the *N* 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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The following coefficients must be defined for each angle type via the
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:doc:`angle_coeff <angle_coeff>` command as in the example above.
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* filename
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* keyword
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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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----------
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The format of a tabulated file is as follows (without the
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parenthesized comments):
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.. parsed-literal::
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# Angle potential for harmonic (one or more comment or blank lines)
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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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A section begins with a non-blank line whose first 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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:doc:`angle_coeff <angle_coeff>` 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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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 *N*
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specified in the :doc:`angle_style table <angle_style>` command. Let
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Ntable = *N* 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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The "FP" parameter is optional. If used, it is followed by two values
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fplo and fphi, which are the second 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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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 :doc:`fix shake <fix_shake>` command. If not used, the equilibrium angle is
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set to 180.0.
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Following a blank line, the next N lines list the tabulated values.
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On each line, the first value is the index from 1 to N, the second value is
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the angle value (in degrees), the third value is the energy (in energy
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units), and the fourth is -dE/d(theta) (also in energy units). The third
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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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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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----------
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.. include:: accel_styles.rst
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----------
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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This angle style writes the settings for the "angle_style table"
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command to :doc:`binary restart files <restart>`, so a angle_style
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command does not need to specified in an input script that reads a
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restart file. However, the coefficient information is not stored in
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the restart file, since it is tabulated in the potential files. Thus,
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angle_coeff commands do need to be specified in the restart input
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script.
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Restrictions
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""""""""""""
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This angle style can only be used if LAMMPS was built with the
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MOLECULE package. See the :doc:`Build package <Build_package>` doc page
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for more info.
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Related commands
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""""""""""""""""
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:doc:`angle_coeff <angle_coeff>`
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Default
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"""""""
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none
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