lammps/doc/fix_gle.txt

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fix gle command :h3

[Syntax:]

fix ID id-group gle Ns Tstart Tstop seed Amatrix [noneq Cmatrix] [every stride] :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
gle = style name of this fix command :l
Ns = number of additional fictitious momenta :l
Tstart, Tstop = temperature ramp during the run :l
Amatrix = file to read the drift matrix A from :l
seed = random number seed to use for generating noise (positive integer) :l
zero or more keyword/value pairs may be appended :l
keyword = {noneq} and/or {every}
  {noneq} Cmatrix  = file to read the non-equilibrium covariance matrix from 
  {every} stride   = apply the GLE once every time steps. Reduces the accuracy
      of the integration of the GLE, but has *no effect* on the accuracy of equilibrium
      sampling. It might change sampling properties when used together with {noneq}.
:ule

[Examples:]

fix 3 boundary gle 6 300 300 31415 smart.A
fix 1 all gle 6 300 300 31415 qt-300k.A noneq qt-300k.C

[Description:]

Apply a Generalized Langevin Equation (GLE) thermostat as described
in "(Ceriotti)"_#Ceriotti. The formalism allows one to obtain a number
of different effects ranging from efficient sampling of all 
vibrational modes in the system to inexpensive (approximate) 
modelling of nuclear quantum effects. Contrary to 
"fix langevin"_fix_langevin.html, this fix performs both 
thermostatting and evolution of the Hamiltonian equations of motion, so it 
does not need to be used together with "fix nve"_fix_nve.html.

Each degree of freedom in the thermostatted group is supplemented
with Ns additional degrees of freedom s, and the equations of motion
become

dq/dt=p/m
d(p,s)/dt=(F,0) - A(p,s) + B dW/dt

where F is the physical force, A is the drift matrix (that generalizes 
the friction in Langevin dynamics), B is the diffusion term and dW/dt 
un-correlated Gaussian random forces. The A matrix couples the physical
(q,p) dynamics with that of the additional degrees of freedom, 
and makes it possible to obtain effectively a history-dependent 
noise and friction kernel. 

The drift matrix should be given as an external file {Afile},
as a (Ns+1 x Ns+1) matrix in inverse time units. Matrices that are
optimal for a given application and the system of choice can be 
obtained from "(GLE4MD)"_#GLE4MD.

Equilibrium sampling a temperature T is obtained by specifiying the 
target value as the {Tstart} and {Tstop} arguments, so that the diffusion 
matrix that gives canonical sampling for a given A is computed automatically.
However, the GLE framework also allow for non-equilibrium sampling, that
can be used for instance to model inexpensively zero-point energy
effects "(Ceriotti2)"_#Ceriotti2. This is achieved specifying the 
{noneq} keyword followed by the name of the file that contains the
static covariance matrix for the non-equilibrium dynamics.

Since integrating GLE dynamics can be costly when used together with 
simple potentials, one can use the {every} optional keyword to 
apply the Langevin terms only once every several MD steps, in a 
multiple time-step fashion. This should be used with care when doing
non-equilibrium sampling, but should have no effect on equilibrium
averages when using canonical sampling.

The random number {seed} must be a positive integer.  A Marsaglia random
number generator is used.  Each processor uses the input seed to
generate its own unique seed and its own stream of random numbers.
Thus the dynamics of the system will not be identical on two runs on
different numbers of processors. 

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

The instantaneous values of the extended variables are written to
"binary restart files"_restart.html.  Because the state of the random
number generator is not saved in restart files, this means you cannot
do "exact" restarts with this fix, where the simulation continues on
the same as if no restart had taken place. However, in a statistical
sense, a restarted simulation should produce the same behavior.
Note however that you should use a different seed each time you
restart, otherwise the same sequence of random numbers will be used 
each time, which might lead to stochastic synchronization and 
subtle artefacts in the sampling. 

This fix can ramp its target temperature over multiple runs, using the
{start} and {stop} keywords of the "run"_run.html command.  See the
"run"_run.html command for details of how to do this.

The "fix_modify"_fix_modify.html {energy} option is supported by this
fix to add the energy change induced by Langevin thermostatting to the
system's potential energy as part of "thermodynamic
output"_thermo_style.html.  

This fix computes a global scalar which can be accessed by various
"output commands"_Section_howto.html#howto_15.  The scalar is the
cummulative energy change due to this fix.  The scalar value
calculated by this fix is "extensive".  

[Restrictions:] 

The GLE thermostat in its current implementation should not be used
with rigid bodies, SHAKE or RATTLE. It is expected that all the
thermostatted degrees of freedom are fully flexible, and the sampled
ensemble will not be correct otherwise.

This fix is part of the USER-MISC package.  It is only enabled if LAMMPS
was built with that package.  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info.

[Related commands:]

"fix nvt"_fix_nh.html, "fix temp/rescale"_fix_temp_rescale.html, "fix
viscous"_fix_viscous.html, "fix nvt"_fix_nh.html, "pair_style
dpd/tstat"_pair_dpd.html, "fix_gld"_fix_gld.html

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:link(Ceriotti)
[(Ceriotti)] Ceriotti, Bussi and Parrinello, J Chem Theory Comput 6, 
1170-80 (2010)

:link(GLE4MD)
[(GLE4MD)] "http://epfl-cosmo.github.io/gle4md/"_http://epfl-cosmo.github.io/gle4md/

:link(Ceriotti2)
[(Ceriotti2)] Ceriotti, Bussi and Parrinello, Phys Rev Lett 103, 
030603 (2009)