forked from lijiext/lammps
141 lines
5.0 KiB
Plaintext
Executable File
141 lines
5.0 KiB
Plaintext
Executable File
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix ti/rs command :h3
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[Syntax:]
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fix ID group-ID ti/rs lambda_initial lambda_final t_switch t_equil keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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ti/rs = style name of this fix command :l
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lambda_initial/lambda_final = initial/final values of the coupling parameter :l
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t_switch/t_equil = number of steps of the switching/equilibration procedure :l
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keyword = {function} :l
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{function} value = function-ID
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function-ID = ID of the switching function (1, 2 or 3) :pre
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:ule
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[Example:]
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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 :pre
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[Description:]
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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 "(de Koning99,"_#deKoning99 "de
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Koning00a)"_#deKoning00a". The thermodynamic integration is performed
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using the nonequilibrium method of Adiabatic Switching
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"(Watanabe,"_#Watanabe "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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:c,image(Eqs/fix_ti_rs_force.jpg)
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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 "unfix"_unfix.html
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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 "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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:c,image(Eqs/fix_ti_rs_function_1.jpg)
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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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:c,image(Eqs/fix_ti_rs_function_2.jpg)
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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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:c,image(Eqs/fix_ti_rs_function_3.jpg)
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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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No information about this fix is written to "binary restart
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files"_restart.html.
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This fix computes a global vector quantitie which can be accessed by
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various "output commands"_Section_howto.html#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 "run"_run.html command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the "minimize"_minimize.html command.
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[Related commands:]
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"fix ti/spring"_fix_ti_spring.html
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[Restrictions:]
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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 "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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[Default:]
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The keyword default is function = 1.
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:line
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:link(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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:link(Watanabe)
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[(Watanabe)] M. Watanabe and W. P. Reinhardt, Phys Rev Lett, 65, 3301 (1990).
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:link(deKoning96)
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[(de Koning 96)] M. de Koning and A. Antonelli, Phys Rev E, 53, 465 (1996).
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:link(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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:link(deKoning00b)
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[(de Koning 00b)] M. de Koning et al., Computing in Science & Engineering, 2, 88 (2000).
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