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doc/html/_sources/fix_ti_rs.txt
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doc/html/_sources/fix_ti_rs.txt
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.. index:: fix ti/rs
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fix ti/rs command
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=================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID ti/rs lambda_initial lambda_final t_switch t_equil keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* ti/rs = style name of this fix command
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* lambda_initial/lambda_final = initial/final values of the coupling parameter
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* t_switch/t_equil = number of steps of the switching/equilibration procedure
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* keyword = *function*
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.. parsed-literal::
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*function* value = function-ID
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function-ID = ID of the switching function (1, 2 or 3)
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**Example:**
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.. parsed-literal::
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fix ref all ti/rs 50.0 2000 1000
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fix vf vacancy ti/rs 10.0 70000 50000 function 2
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Description
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"""""""""""
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This fix allows you to compute the free energy temperature dependence
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by performing a thermodynamic integration procedure known as
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Reversible Scaling :ref:`(de Koning99, <deKoning99>` :ref:`de Koning00a) <deKoning00a">`. The thermodynamic integration is performed
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using the nonequilibrium method of Adiabatic Switching
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:ref:`(Watanabe, <Watanabe>` :ref:`de Koning96) <deKoning96>`.
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The forces on the atoms are dynamically scaled during the simulation,
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the rescaling is done in the following manner:
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.. image:: Eqs/fix_ti_rs_force.jpg
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:align: center
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where F_int is the total force on the atoms due to the interatomic
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potential and lambda is the coupling parameter of the thermodynamic
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integration.
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The fix acts as follows: during the first *t_equil* steps after the
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fix is defined the value of lambda is *lambda_initial* , this is the
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period to equilibrate the system in the lambda = *lambda_initial*
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state. After this the value of lambda changes continuously from
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*lambda_initial* to *lambda_final* according to the function defined
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using the keyword *function* (described below), this is done in
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*t_switch* steps. Then comes the second equilibration period of
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*t_equil* to equilibrate the system in the lambda = *lambda_final*
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state. After that the switching back to the lambda = *lambda_initial*
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state is done using *t_switch* timesteps and following the same
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switching function. After this period the value of lambda is kept
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equal to *lambda_initial* indefinitely or until a :doc:`unfix <unfix>`
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erase the fix.
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The description of thermodynamic integration in both directions is
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done in :ref:`de Koning00b <deKoning00b>`, the main reason is to try to
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eliminate the dissipated heat due to the nonequilibrium process.
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The *function* keyword allows the use of three different switching
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rates. The option *1* results in a constant rescaling where the lambda
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parameter changes at a constant rate during the switching time
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according to the switching function
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.. image:: Eqs/fix_ti_rs_function_1.jpg
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:align: center
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where tau is the scaled time variable t/t_switch. This switching
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function has the characteristic that the temperature scaling is faster
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at temperatures closer to the final temperature of the procedure. The
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option number *2* performs the switching at a rate defined by the
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following switching function
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.. image:: Eqs/fix_ti_rs_function_2.jpg
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:align: center
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This switching function has the characteristic that the temperature
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scaling occurs at a constant rate during all the procedure. The option
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number *3* performs the switching at a rate defined by the following
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switching function
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.. image:: Eqs/fix_ti_rs_function_3.jpg
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:align: center
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This switching function has the characteristic that the temperature
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scaling is faster at temperatures closer to the initial temperature of
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the procedure.
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An example script using this command is provided in the
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examples/USER/misc/ti directory.
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Restart, fix_modify, output, run start/stop, minimize info
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""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files <restart>`.
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This fix computes a global vector quantitie which can be accessed by
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various :ref:`output commands <howto_15>`. The vector has
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2 positions, the first one is the coupling parameter lambda and the
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second one is the time derivative of lambda. The scalar and vector
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values calculated by this fix are "extensive".
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the :doc:`minimize <minimize>` command.
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Related commands
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""""""""""""""""
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:doc:`fix ti/spring <fix_ti_spring>`
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Restrictions
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""""""""""""
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This command is part of the USER-MISC package. It is only enabled if
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LAMMPS was built with those packages. See the :ref:`Making LAMMPS <start_3>` section for more info.
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Default
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"""""""
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The keyword default is function = 1.
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----------
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.. _deKoning99:
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**(de Koning 99)** M. de Koning, A. Antonelli and S. Yip, Phys Rev Lett, 83, 3973 (1999).
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.. _Watanabe:
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**(Watanabe)** M. Watanabe and W. P. Reinhardt, Phys Rev Lett, 65, 3301 (1990).
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.. _deKoning96:
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**(de Koning 96)** M. de Koning and A. Antonelli, Phys Rev E, 53, 465 (1996).
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.. _deKoning00a:
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**(de Koning 00a)** M. de Koning, A. Antonelli and S. Yip, J Chem Phys, 115, 11025 (2000).
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.. _deKoning00b:
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**(de Koning 00b)** M. de Koning et al., Computing in Science & Engineering, 2, 88 (2000).
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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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