.. index:: pair_style tersoff/mod pair_style tersoff/mod command ============================== pair_style tersoff/mod/gpu command ================================== pair_style tersoff/mod/kk command ================================= pair_style tersoff/mod/omp command ================================== Syntax """""" .. parsed-literal:: pair_style tersoff/mod Examples """""""" .. parsed-literal:: pair_style tersoff/mod pair_coeff * * Si.tersoff.mod Si Si Description """"""""""" The *tersoff/mod* style computes a bond-order type interatomic potential :ref:`(Kumagai) ` based on a 3-body Tersoff potential :ref:`(Tersoff_1) `, :ref:`(Tersoff_2) ` with modified cutoff function and angular-dependent term, giving the energy E of a system of atoms as .. image:: Eqs/pair_tersoff_mod.jpg :align: center where f_R is a two-body term and f_A includes three-body interactions. The summations in the formula are over all neighbors J and K of atom I within a cutoff distance = R + D. The modified cutoff function f_C proposed by :ref:`(Murty) ` and having a continuous second-order differential is employed. The angular-dependent term g(theta) was modified to increase the flexibility of the potential. The *tersoff/mod* potential is fitted to both the elastic constants and melting point by employing the modified Tersoff potential function form in which the angular-dependent term is improved. The model performs extremely well in describing the crystalline, liquid, and amorphous phases :ref:`(Schelling) `. Only a single pair_coeff command is used with the *tersoff/mod* style which specifies a Tersoff/MOD potential file with parameters for all needed elements. These are mapped to LAMMPS atom types by specifying N additional arguments after the filename in the pair_coeff command, where N is the number of LAMMPS atom types: * filename * N element names = mapping of Tersoff/MOD elements to atom types As an example, imagine the Si.tersoff_mod file has Tersoff values for Si. If your LAMMPS simulation has 3 Si atoms types, you would use the following pair_coeff command: .. parsed-literal:: pair_coeff * * Si.tersoff_mod Si Si Si The 1st 2 arguments must be * * so as to span all LAMMPS atom types. The three Si arguments map LAMMPS atom types 1,2,3 to the Si element in the Tersoff/MOD file. If a mapping value is specified as NULL, the mapping is not performed. This can be used when a *tersoff/mod* potential is used as part of the *hybrid* pair style. The NULL values are placeholders for atom types that will be used with other potentials. Tersoff/MOD file in the *potentials* directory of the LAMMPS distribution have a ".tersoff.mod" suffix. Lines that are not blank or comments (starting with #) define parameters for a triplet of elements. The parameters in a single entry correspond to coefficients in the formula above: * element 1 (the center atom in a 3-body interaction) * element 2 (the atom bonded to the center atom) * element 3 (the atom influencing the 1-2 bond in a bond-order sense) * beta * alpha * h * eta * beta_ters = 1 (dummy parameter) * lambda2 (1/distance units) * B (energy units) * R (distance units) * D (distance units) * lambda1 (1/distance units) * A (energy units) * n * c1 * c2 * c3 * c4 * c5 The n, eta, lambda2, B, lambda1, and A parameters are only used for two-body interactions. The beta, alpha, c1, c2, c3, c4, c5, h parameters are only used for three-body interactions. The R and D parameters are used for both two-body and three-body interactions. The non-annotated parameters are unitless. The Tersoff/MOD potential file must contain entries for all the elements listed in the pair_coeff command. It can also contain entries for additional elements not being used in a particular simulation; LAMMPS ignores those entries. For a single-element simulation, only a single entry is required (e.g. SiSiSi). As annotated above, the first element in the entry is the center atom in a three-body interaction and it is bonded to the 2nd atom and the bond is influenced by the 3rd atom. Thus an entry for SiSiSi means Si bonded to a Si with another Si atom influencing the bond. ---------- Styles with a *cuda*\ , *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed in :doc:`Section_accelerate ` of the manual. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues. These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL, KOKKOS, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the :ref:`Making LAMMPS ` section for more info. You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the :ref:`-suffix command-line switch ` when you invoke LAMMPS, or you can use the :doc:`suffix ` command in your input script. See :doc:`Section_accelerate ` of the manual for more instructions on how to use the accelerated styles effectively. ---------- **Mixing, shift, table, tail correction, restart, rRESPA info**\ : This pair style does not support the :doc:`pair_modify ` shift, table, and tail options. This pair style does not write its information to :doc:`binary restart files `, since it is stored in potential files. Thus, you need to re-specify the pair_style and pair_coeff commands in an input script that reads a restart file. This pair style can only be used via the *pair* keyword of the :doc:`run_style respa ` command. It does not support the *inner*\ , *middle*\ , *outer* keywords. ---------- Restrictions """""""""""" This pair style is part of the MANYBODY package. It is only enabled if LAMMPS was built with that package (which it is by default). See the :ref:`Making LAMMPS ` section for more info. This pair style requires the :doc:`newton ` setting to be "on" for pair interactions. The Tersoff/MOD potential files provided with LAMMPS (see the potentials directory) are parameterized for metal :doc:`units `. You can use the Tersoff/MOD potential with any LAMMPS units, but you would need to create your own Tersoff/MOD potential file with coefficients listed in the appropriate units if your simulation doesn't use "metal" units. Related commands """""""""""""""" :doc:`pair_coeff ` **Default:** none ---------- .. _Kumagai: **(Kumagai)** T. Kumagai, S. Izumi, S. Hara, S. Sakai, Comp. Mat. Science, 39, 457 (2007). .. _Tersoff_1: **(Tersoff_1)** J. Tersoff, Phys Rev B, 37, 6991 (1988). .. _Tersoff_2: **(Tersoff_2)** J. Tersoff, Phys Rev B, 38, 9902 (1988). .. _Murty: **(Murty)** M.V.R. Murty, H.A. Atwater, Phys Rev B, 51, 4889 (1995). .. _Schelling: **(Schelling)** Patrick K. Schelling, Comp. Mat. Science, 44, 274 (2008). .. _lws: http://lammps.sandia.gov .. _ld: Manual.html .. _lc: Section_commands.html#comm