forked from lijiext/lammps
343 lines
13 KiB
Plaintext
343 lines
13 KiB
Plaintext
.. index:: pair_style reax/c
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pair_style reax/c command
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=========================
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Syntax
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""""""
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.. parsed-literal::
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pair_style reax/c cfile keyword value
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* cfile = NULL or name of a control file
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* zero or more keyword/value pairs may be appended
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.. parsed-literal::
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keyword = *checkqeq* or *lgvdw* or *safezone* or *mincap*
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*checkqeq* value = *yes* or *no* = whether or not to require qeq/reax fix
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*lgvdw* value = *yes* or *no* = whether or not to use a low gradient vdW correction
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*safezone* = factor used for array allocation
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*mincap* = minimum size for array allocation
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Examples
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""""""""
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.. parsed-literal::
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pair_style reax/c NULL
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pair_style reax/c controlfile checkqeq no
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pair_style reax/c NULL lgvdw yes
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pair_style reax/c NULL safezone 1.6 mincap 100
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pair_coeff * * ffield.reax C H O N
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Description
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"""""""""""
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Style *reax/c* computes the ReaxFF potential of van Duin, Goddard and
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co-workers. ReaxFF uses distance-dependent bond-order functions to
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represent the contributions of chemical bonding to the potential
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energy. There is more than one version of ReaxFF. The version
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implemented in LAMMPS uses the functional forms documented in the
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supplemental information of the following paper: :ref:`(Chenoweth et al., 2008) <Chenoweth_2008>`. The version integrated into LAMMPS matches
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the most up-to-date version of ReaxFF as of summer 2010. For more
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technical details about the pair reax/c implementation of ReaxFF, see
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the :ref:`(Aktulga) <Aktulga>` paper.
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The *reax/c* style differs from the :doc:`pair_style reax <pair_reax>`
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command in the lo-level implementation details. The *reax* style is a
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Fortran library, linked to LAMMPS. The *reax/c* style was initially
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implemented as stand-alone C code and is now integrated into LAMMPS as
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a package.
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LAMMPS provides several different versions of ffield.reax in its
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potentials dir, each called potentials/ffield.reax.label. These are
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documented in potentials/README.reax. The default ffield.reax
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contains parameterizations for the following elements: C, H, O, N.
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The format of these files is identical to that used originally by van
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Duin. We have tested the accuracy of *pair_style reax/c* potential
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against the original ReaxFF code for the systems mentioned above. You
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can use other ffield files for specific chemical systems that may be
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available elsewhere (but note that their accuracy may not have been
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tested).
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.. note::
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We do not distribute a wide variety of ReaxFF force field files
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with LAMMPS. Adri van Duin's group at PSU is the central repository
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for this kind of data as they are continuously deriving and updating
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parameterizations for different classes of materials. You can submit
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a contact request at the Materials Computation Center (MCC) website
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`https://www.mri.psu.edu/materials-computation-center/connect-mcc <https://www.mri.psu.edu/materials-computation-center/connect-mcc>`_,
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describing the material(s) you are interested in modeling with ReaxFF.
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They can tell
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you what is currently available or what it would take to create a
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suitable ReaxFF parameterization.
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The *cfile* setting can be specified as NULL, in which case default
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settings are used. A control file can be specified which defines
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values of control variables. Some control variables are
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global parameters for the ReaxFF potential. Others define certain
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performance and output settings.
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Each line in the control file specifies the value for
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a control variable. The format of the control file is described
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below.
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.. note::
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The LAMMPS default values for the ReaxFF global parameters
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correspond to those used by Adri van Duin's stand-alone serial
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code. If these are changed by setting control variables in the control
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file, the results from LAMMPS and the serial code will not agree.
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Two examples using *pair_style reax/c* are provided in the examples/reax
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sub-directory, along with corresponding examples for
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:doc:`pair_style reax <pair_reax>`.
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Use of this pair style requires that a charge be defined for every
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atom. See the :doc:`atom_style <atom_style>` and
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:doc:`read_data <read_data>` commands for details on how to specify
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charges.
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The ReaxFF parameter files provided were created using a charge
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equilibration (QEq) model for handling the electrostatic interactions.
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Therefore, by default, LAMMPS requires that the :doc:`fix qeq/reax <fix_qeq_reax>` command be used with *pair_style reax/c*
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when simulating a ReaxFF model, to equilibrate charge each timestep.
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Using the keyword *checkqeq* with the value *no*
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turns off the check for *fix qeq/reax*,
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allowing a simulation to be run without charge equilibration.
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In this case, the static charges you
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assign to each atom will be used for computing the electrostatic
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interactions in the system.
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See the :doc:`fix qeq/reax <fix_qeq_reax>` command for details.
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Using the optional keyword *lgvdw* with the value *yes* turns on
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the low-gradient correction of the ReaxFF/C for long-range
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London Dispersion, as described in the :ref:`(Liu) <Liu_2011>` paper. Force field
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file *ffield.reax.lg* is designed for this correction, and is trained
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for several energetic materials (see "Liu"). When using lg-correction,
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recommended value for parameter *thb* is 0.01, which can be set in the
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control file. Note: Force field files are different for the original
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or lg corrected pair styles, using wrong ffield file generates an error message.
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Optional keywords *safezone* and *mincap* are used for allocating
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reax/c arrays. Increase these values can avoid memory problems, such
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as segmentation faults and bondchk failed errors, that could occur under
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certain conditions.
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The thermo variable *evdwl* stores the sum of all the ReaxFF potential
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energy contributions, with the exception of the Coulombic and charge
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equilibration contributions which are stored in the thermo variable
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*ecoul*. The output of these quantities is controlled by the
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:doc:`thermo <thermo>` command.
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This pair style tallies a breakdown of the total ReaxFF potential
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energy into sub-categories, which can be accessed via the :doc:`compute pair <compute_pair>` command as a vector of values of length 14.
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The 14 values correspond to the following sub-categories (the variable
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names in italics match those used in the original FORTRAN ReaxFF code):
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1. *eb* = bond energy
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2. *ea* = atom energy
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3. *elp* = lone-pair energy
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4. *emol* = molecule energy (always 0.0)
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5. *ev* = valence angle energy
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6. *epen* = double-bond valence angle penalty
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7. *ecoa* = valence angle conjugation energy
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8. *ehb* = hydrogen bond energy
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9. *et* = torsion energy
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10. *eco* = conjugation energy
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11. *ew* = van der Waals energy
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12. *ep* = Coulomb energy
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13. *efi* = electric field energy (always 0.0)
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14. *eqeq* = charge equilibration energy
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To print these quantities to the log file (with descriptive column
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headings) the following commands could be included in an input script:
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.. parsed-literal::
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compute reax all pair reax/c
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variable eb equal c_reax[1]
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variable ea equal c_reax[2]
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...
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variable eqeq equal c_reax[14]
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thermo_style custom step temp epair v_eb v_ea ... v_eqeq
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Only a single pair_coeff command is used with the *reax/c* style which
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specifies a ReaxFF potential file with parameters for all needed
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elements. These are mapped to LAMMPS atom types by specifying N
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additional arguments after the filename in the pair_coeff command,
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where N is the number of LAMMPS atom types:
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* filename
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* N indices = ReaxFF elements
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The filename is the ReaxFF potential file. Unlike for the *reax*
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pair style, any filename can be used.
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In the ReaxFF potential file, near the top, after the general
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parameters, is the atomic parameters section that contains element
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names, each with a couple dozen numeric parameters. If there are M
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elements specified in the *ffield* file, think of these as numbered 1
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to M. Each of the N indices you specify for the N atom types of LAMMPS
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atoms must be an integer from 1 to M. Atoms with LAMMPS type 1 will
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be mapped to whatever element you specify as the first index value,
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etc. If a mapping value is specified as NULL, the mapping is not
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performed. This can be used when the *reax/c* style is used as part
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of the *hybrid* pair style. The NULL values are placeholders for atom
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types that will be used with other potentials.
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As an example, say your LAMMPS simulation has 4 atom types and the
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elements are ordered as C, H, O, N in the *ffield* file. If you want
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the LAMMPS atom type 1 and 2 to be C, type 3 to be N, and type 4 to be
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H, you would use the following pair_coeff command:
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.. parsed-literal::
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pair_coeff * * ffield.reax C C N H
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----------
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The format of a line in the control file is as follows:
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.. parsed-literal::
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variable_name value
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and it may be followed by an "!" character and a trailing comment.
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If the value of a control variable is not specified, then default
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values are used. What follows is the list of variables along with a
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brief description of their use and default values.
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simulation_name: Output files produced by *pair_style reax/c* carry
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this name + extensions specific to their contents. Partial energies
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are reported with a ".pot" extension, while the trajectory file has
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".trj" extension.
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tabulate_long_range: To improve performance, long range interactions
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can optionally be tabulated (0 means no tabulation). Value of this
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variable denotes the size of the long range interaction table. The
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range from 0 to long range cutoff (defined in the *ffield* file) is
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divided into *tabulate_long_range* points. Then at the start of
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simulation, we fill in the entries of the long range interaction table
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by computing the energies and forces resulting from van der Waals and
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Coulomb interactions between every possible atom type pairs present in
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the input system. During the simulation we consult to the long range
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interaction table to estimate the energy and forces between a pair of
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atoms. Linear interpolation is used for estimation. (default value =
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0)
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energy_update_freq: Denotes the frequency (in number of steps) of
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writes into the partial energies file. (default value = 0)
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nbrhood_cutoff: Denotes the near neighbors cutoff (in Angstroms)
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regarding the bonded interactions. (default value = 5.0)
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hbond_cutoff: Denotes the cutoff distance (in Angstroms) for hydrogen
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bond interactions.(default value = 7.5. Value of 0.0 turns off hydrogen
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bonds)
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bond_graph_cutoff: is the threshold used in determining what is a
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physical bond, what is not. Bonds and angles reported in the
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trajectory file rely on this cutoff. (default value = 0.3)
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thb_cutoff: cutoff value for the strength of bonds to be considered in
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three body interactions. (default value = 0.001)
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thb_cutoff_sq: cutoff value for the strength of bond order products
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to be considered in three body interactions. (default value = 0.00001)
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write_freq: Frequency of writes into the trajectory file. (default
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value = 0)
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traj_title: Title of the trajectory - not the name of the trajectory
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file.
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atom_info: 1 means print only atomic positions + charge (default = 0)
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atom_forces: 1 adds net forces to atom lines in the trajectory file
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(default = 0)
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atom_velocities: 1 adds atomic velocities to atoms line (default = 0)
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bond_info: 1 prints bonds in the trajectory file (default = 0)
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angle_info: 1 prints angles in the trajectory file (default = 0)
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----------
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**Mixing, shift, table, tail correction, restart, rRESPA info**:
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This pair style does not support the :doc:`pair_modify <pair_modify>`
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mix, shift, table, and tail options.
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This pair style does not write its information to :doc:`binary restart files <restart>`, since it is stored in potential files. Thus, you
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need to re-specify the pair_style and pair_coeff commands in an input
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script that reads a restart file.
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This pair style can only be used via the *pair* keyword of the
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:doc:`run_style respa <run_style>` command. It does not support the
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*inner*, *middle*, *outer* keywords.
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Restrictions
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""""""""""""
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This pair style is part of the USER-REAXC package. It is only enabled
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if LAMMPS was built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
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The ReaxFF potential files provided with LAMMPS in the potentials
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directory are parameterized for real :doc:`units <units>`. You can use
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the ReaxFF potential with any LAMMPS units, but you would need to
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create your own potential file with coefficients listed in the
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appropriate units if your simulation doesn't use "real" units.
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Related commands
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""""""""""""""""
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:doc:`pair_coeff <pair_coeff>`, :doc:`fix qeq/reax <fix_qeq_reax>`, :doc:`fix reax/c/bonds <fix_reax_bonds>`, :doc:`fix reax/c/species <fix_reaxc_species>`, :doc:`pair_style reax <pair_reax>`
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Default
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"""""""
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The keyword defaults are checkqeq = yes, lgvdw = no, safezone = 1.2,
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mincap = 50.
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----------
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.. _Chenoweth_2008:
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**(Chenoweth_2008)** Chenoweth, van Duin and Goddard,
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Journal of Physical Chemistry A, 112, 1040-1053 (2008).
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.. _Aktulga:
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(Aktulga) Aktulga, Fogarty, Pandit, Grama, Parallel Computing, 38,
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245-259 (2012).
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.. _Liu_2011:
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**(Liu)** L. Liu, Y. Liu, S. V. Zybin, H. Sun and W. A. Goddard, Journal
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of Physical Chemistry A, 115, 11016-11022 (2011).
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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