-
- -
-

fix ti/rs command

-
-

Syntax

-
-fix ID group-ID ti/rs lambda_initial lambda_final t_switch t_equil keyword value ...
-
-
    -
  • ID, group-ID are documented in fix command
  • -
  • ti/rs = style name of this fix command
  • -
  • lambda_initial/lambda_final = initial/final values of the coupling parameter
  • -
  • t_switch/t_equil = number of steps of the switching/equilibration procedure
  • -
  • keyword = function
  • -
-
-function value = function-ID
-  function-ID = ID of the switching function (1, 2 or 3)
-
-

Example:

-
fix ref all ti/rs 50.0 2000 1000
-fix vf vacancy ti/rs 10.0 70000 50000 function 2
-
-
-
-
-

Description

-

This fix allows you to compute the free energy temperature dependence -by performing a thermodynamic integration procedure known as -Reversible Scaling (de Koning99, de Koning00a). The thermodynamic integration is performed -using the nonequilibrium method of Adiabatic Switching -(Watanabe, de Koning96).

-

The forces on the atoms are dynamically scaled during the simulation, -the rescaling is done in the following manner:

-_images/fix_ti_rs_force.jpg -

where F_int is the total force on the atoms due to the interatomic -potential and lambda is the coupling parameter of the thermodynamic -integration.

-

The fix acts as follows: during the first t_equil steps after the -fix is defined the value of lambda is lambda_initial , this is the -period to equilibrate the system in the lambda = lambda_initial -state. After this the value of lambda changes continuously from -lambda_initial to lambda_final according to the function defined -using the keyword function (described below), this is done in -t_switch steps. Then comes the second equilibration period of -t_equil to equilibrate the system in the lambda = lambda_final -state. After that the switching back to the lambda = lambda_initial -state is done using t_switch timesteps and following the same -switching function. After this period the value of lambda is kept -equal to lambda_initial indefinitely or until a unfix -erase the fix.

-

The description of thermodynamic integration in both directions is -done in de Koning00b, the main reason is to try to -eliminate the dissipated heat due to the nonequilibrium process.

-

The function keyword allows the use of three different switching -rates. The option 1 results in a constant rescaling where the lambda -parameter changes at a constant rate during the switching time -according to the switching function

-_images/fix_ti_rs_function_1.jpg -

where tau is the scaled time variable t/t_switch. This switching -function has the characteristic that the temperature scaling is faster -at temperatures closer to the final temperature of the procedure. The -option number 2 performs the switching at a rate defined by the -following switching function

-_images/fix_ti_rs_function_2.jpg -

This switching function has the characteristic that the temperature -scaling occurs at a constant rate during all the procedure. The option -number 3 performs the switching at a rate defined by the following -switching function

-_images/fix_ti_rs_function_3.jpg -

This switching function has the characteristic that the temperature -scaling is faster at temperatures closer to the initial temperature of -the procedure.

-

An example script using this command is provided in the -examples/USER/misc/ti directory.

-

Restart, fix_modify, output, run start/stop, minimize info:

-

No information about this fix is written to binary restart files.

-

This fix computes a global vector quantitie which can be accessed by -various output commands. The vector has -2 positions, the first one is the coupling parameter lambda and the -second one is the time derivative of lambda. The scalar and vector -values calculated by this fix are “extensive”.

-

No parameter of this fix can be used with the start/stop keywords of -the run command.

-

The forces due to this fix are imposed during an energy minimization, -invoked by the minimize command.

-
- -
-

Restrictions

-

This command is part of the USER-MISC package. It is only enabled if -LAMMPS was built with those packages. See the Making LAMMPS section for more info.

-
-
-

Default

-

The keyword default is function = 1.

-
-

(de Koning 99) M. de Koning, A. Antonelli and S. Yip, Phys Rev Lett, 83, 3973 (1999).

-

(Watanabe) M. Watanabe and W. P. Reinhardt, Phys Rev Lett, 65, 3301 (1990).

-

(de Koning 96) M. de Koning and A. Antonelli, Phys Rev E, 53, 465 (1996).

-

(de Koning 00a) M. de Koning, A. Antonelli and S. Yip, J Chem Phys, 115, 11025 (2000).

-

(de Koning 00b) M. de Koning et al., Computing in Science & Engineering, 2, 88 (2000).

-
-
- - -
-