144 lines
4.5 KiB
ReStructuredText
144 lines
4.5 KiB
ReStructuredText
.. index:: pair_style ufm
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.. index:: pair_style ufm/gpu
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.. index:: pair_style ufm/omp
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.. index:: pair_style ufm/opt
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pair_style ufm command
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======================
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Accelerator Variants: *ufm/gpu*, *ufm/omp*, *ufm/opt*
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Syntax
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""""""
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.. code-block:: LAMMPS
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pair_style ufm cutoff
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* cutoff = global cutoff for *ufm* interactions (distance units)
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Examples
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""""""""
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.. code-block:: LAMMPS
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pair_style ufm 4.0
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pair_coeff 1 1 100.0 1.0 2.5
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pair_coeff * * 100.0 1.0
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pair_style ufm 4.0
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pair_coeff * * 10.0 1.0
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variable prefactor equal ramp(10,100)
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fix 1 all adapt 1 pair ufm epsilon * * v_prefactor
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Description
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"""""""""""
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Style *ufm* computes pairwise interactions using the Uhlenbeck-Ford model (UFM) potential :ref:`(Paula Leite2016) <PL2>` which is given by
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.. math::
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E & = -\varepsilon\, \ln{\left[1-\exp{\left(-r^{2}/\sigma^{2}\right)}\right]} \qquad r < r_c \\
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\varepsilon & = p\,k_B\,T
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where :math:`r_c` is the cutoff, :math:`\sigma` is a distance-scale and
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:math:`\epsilon` is an energy-scale, i.e., a product of Boltzmann constant
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:math:`k_B`, temperature *T* and the Uhlenbeck-Ford p-parameter which
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is responsible
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to control the softness of the interactions :ref:`(Paula Leite2017) <PL1>`.
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This model is useful as a reference system for fluid-phase free-energy calculations :ref:`(Paula Leite2016) <PL2>`.
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The following coefficients must be defined for each pair of atom types
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via the :doc:`pair_coeff <pair_coeff>` command as in the examples above,
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or in the data file or restart files read by the
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:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
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commands, or by mixing as described below:
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* :math:`\epsilon` (energy units)
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* :math:`\sigma` (distance units)
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* cutoff (distance units)
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The last coefficient is optional. If not specified, the global *ufm*
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cutoff is used.
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The :doc:`fix adapt <fix_adapt>` command can be used to vary epsilon and sigma for this pair style over the course of a simulation, in which case
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pair_coeff settings for epsilon and sigma must still be specified, but will be
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overridden. For example these commands will vary the prefactor epsilon for
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all pairwise interactions from 10.0 at the beginning to 100.0 at the end
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of a run:
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.. code-block:: LAMMPS
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variable prefactor equal ramp(10,100)
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fix 1 all adapt 1 pair ufm epsilon * * v_prefactor
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.. note::
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The thermodynamic integration procedure can be performed with this
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potential using :doc:`fix adapt <fix_adapt>`. This command will
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rescale the force on each atom by varying a scale variable, which
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always starts with value 1.0. The syntax is the same described above,
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however, changing epsilon to scale. A detailed explanation of how to
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use this command and perform nonequilibrium thermodynamic integration
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in LAMMPS is given in the paper by :ref:`(Freitas) <Freitas2>`.
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----------
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.. include:: accel_styles.rst
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----------
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Mixing, shift, table, tail correction, restart, rRESPA info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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For atom type pairs I,J and I != J, the epsilon and sigma coefficients and cutoff
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distance for this pair style can be mixed. The default mix value is *geometric*\ . See the
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"pair_modify" command for details.
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This pair style support the :doc:`pair_modify <pair_modify>` shift option for the energy of the pair interaction.
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The :doc:`pair_modify <pair_modify>` table and tail are not relevant for this
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pair style.
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This pair style does not support the :doc:`pair_modify <pair_modify>` tail option for adding long-range tail corrections to energy and pressure.
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This pair style writes its information to :doc:`binary restart files <restart>`, so pair_style and pair_coeff commands do not need
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to be specified in an input 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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----------
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Restrictions
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""""""""""""
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This pair style is part of the EXTRA-PAIR package. It is only enabled if
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LAMMPS was built with that package. See the
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:doc:`Build package <Build_package>` page for more info.
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Related commands
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""""""""""""""""
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:doc:`pair_coeff <pair_coeff>`, :doc:`fix adapt <fix_adapt>`
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Default
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"""""""
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none
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.. _PL1:
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**(Paula Leite2017)** Paula Leite, Santos-Florez, and de Koning, Phys Rev E, 96,
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32115 (2017).
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.. _PL2:
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**(Paula Leite2016)** Paula Leite , Freitas, Azevedo, and de Koning, J Chem Phys, 126,
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044509 (2016).
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.. _Freitas2:
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**(Freitas)** Freitas, Asta, and de Koning, Computational Materials Science, 112, 333 (2016).
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