lammps/doc/minimize.txt

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minimize command :h3

[Syntax:]

minimize etol ftol maxiter maxeval :pre

etol = stopping tolerance for energy (unitless)
ftol = stopping tolerance for force (force units)
maxiter = max iterations of minimizer
maxeval = max number of force/energy evaluations :ul

[Examples:]

minimize 1.0e-4 1.0e-6 100 1000
minimize 0.0 1.0e-8 1000 100000 :pre

[Description:]

Perform an energy minimization of the system, by iteratively adjusting
atom coordinates.  Iterations are terminated when one of the stopping
criteria is satisfied.  At that point the configuration will hopefully
be in local potential energy minimum.  More precisely, the
configuration should approximate a critical point for the objective
function (see below), which may or may not be a local minimum.

The minimization algorithm used is set by the
"min_style"_min_style.html command.  Other options are set by the
"min_modify"_min_modify.html command.  Minimize commands can be
interspersed with "run"_run.html commands to alternate between
relaxation and dynamics.  The minimizers bound the distance atoms move
in one iteration, so that you can relax systems with highly overlapped
atoms (large energies and forces) by pushing the atoms off of each
other.

Alternate means of relaxing a system are to run dynamics with a small
or "limited timestep"_fix_nve_limit.html.  Or dynamics can be run
using "fix viscous"_fix_viscous.html to impose a damping force that
slowly drains all kinetic energy from the system.  The "pair_style
soft"_pair_soft.html potential can be used to un-overlap atoms while
running dynamics.

A minimization involves an outer iteration loop which sets the search
direction along which atom coordinates are changed.  An inner
iteration is then performed using a line search algorithm.  The line
search typically evaluates forces and energies several times to set
new coordinates.  Currently, a backtracking algorithm is used which
may not be optimal in terms of the number of force evaulations
performed, but appears to be more robust than previous line searches
we've tried.  The backtracking method is described in Nocedal and
Wright's Numerical Optimization (Procedure 3.1 on p 41).

The objective function being minimized is the total potential energy
of the system as a function of the N atom coordinates:

:c,image(Eqs/min_energy.jpg)

where the first term is the sum of all non-bonded "pairwise
interactions"_pair_style.html including "long-range Coulombic
interactions"_kspace_style.html, the 2nd thru 5th terms are
"bond"_bond_style.html, "angle"_angle_style.html,
"dihedral"_dihedral_style.html, and "improper"_improper_style.html
interactions respectively, and the last term is energy due to
"fixes"_fix.html which can act as constraints or apply force to atoms,
such as thru interaction with a wall.  See the discussion below about
how fix commands affect minimization.

The starting point for the minimization is the current configuration
of the atoms.

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The minimization procedure stops if any of several criteria are met:

the change in energy between outer iterations is less than {etol}
the 2-norm (length) of the global force vector is less than the {ftol}
the line search fails because the step distance backtracks to 0.0
the number of outer iterations exceeds {maxiter}
the number of total force evaluations exceeds {maxeval} :ul

For the first criterion, the specified energy tolerance {etol} is
unitless; it is met when the energy change between successive
iterations divided by the energy magnitude is less than or equal to
the tolerance.  For example, a setting of 1.0e-4 for {etol} means an
energy tolerance of one part in 10^4.

For the second criterion, the specified force tolerance {ftol} is in
force units, since it is the length of the global force vector for all
atoms, e.g. a vector of size 3N for N atoms.  Since many of the
components will be near zero after minimization, you can think of
{ftol} as an upper bound on the final force on any component of any
atom.  For example, a setting of 1.0e-4 for {ftol} means no x, y, or z
component of force on any atom will be larger than 1.0e-4 (in force
units) after minimization.

Either or both of the {etol} and {ftol} values can be set to 0.0, in
which case some other criterion will terminate the minimization.

During a minimization, the outer iteration count is treated as a
timestep.  Output is triggered by this timestep, e.g. thermodynamic
output or dump and restart files.

Following minimization, a statistical summary is printed that lists
which convergence criterion caused the minimizer to stop, as well as
information about the energy, force, final line search, and and
iteration counts.  An example is as follows:

Minimization stats:
  Stopping criterion = max iterations
  Energy initial, next-to-last, final = 
       -0.626828169302     -2.82642039062     -2.82643549739
  Force two-norm initial, final = 2052.1 91.9642
  Force max component initial, final = 346.048 9.78056
  Final line search alpha, max atom move = 2.23899e-06 2.18986e-05
  Iterations, force evaluations = 2000 12724 :pre

The 3 energy values are for before and after the minimization and on
the next-to-last iteration.  This is what the {etol} parameter checks.

The two-norm force values are the length of the global force vector
before and after minimization.  This is what the {ftol} parameter
checks.

The max-component force values are the absolute value of the largest
component (x,y,z) in the global force vector.

The alpha parameter for the line-search, when multiplied by the max
force component (on the last iteration), gives the max distance any
atom moved during the last iteration.  Alpha will be 0.0 if the line
search could not reduce the energy.  Even if alpha is non-zero, if the
"max atom move" distance is tiny compared to typical atom coordinates,
then it is possible the last iteration effectively caused no atom
movement and thus the evaluated energy did not change and the
minimizer terminated.  Said another way, even with non-zero forces,
it's possible the effect of those forces is to move atoms a distance
less than machine precision, so that the energy cannot be further
reduced.

The iterations and force evaluation values are what is checked by the
{maxiter} and {maxeval} parameters.

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IMPORTANT NOTE: It is highly recommended that you use a "pair
style"_pair_style.html that goes to 0.0 at the cutoff distance when
performing minimization (even if you later change it when running
dynamics).  If this is not done, the total energy of the system will
have discontinuities when the relative distance between any pair of
atoms changes from cutoff+epsilon to cutoff-epsilon and the minimizer
may behave poorly.

Note that a cutoff Lennard-Jones potential (and others) can be shifted
so that its energy is 0.0 at the cutoff via the
"pair_modify"_pair_modify command.  See the doc pages for inidividual
"pair styles"_pair_style.html for details.  Note that Coulombic
potentials always have a cutoff, unless versions with a long-range
component are used (e.g. "pair_style lj/cut/coul/long"_pair_lj.html).
The CHARMM potentials go to 0.0 at the cutoff (e.g. "pair_style
lj/charmm/coul/charmm"_pair_charmm.html}, as do the GROMACS potentials
(e.g. "pair_style lj/gromacs"_pair_gromacs.html}.

If a soft potential ("pair_style soft"_pair_soft.html) is used the
Astop value is used for the prefactor (no time dependence).

Only some fixes (typically those that apply force constraints) are
invoked during minimization.  See the doc pages for individual
"fix"_fix.html commands to see which ones are relevant.

IMPORTANT NOTE: Some fixes which are invoked during minimization have
an associated potential energy.  For that energy to be included in the
total potential energy of the system (the quantity being minimized),
you MUST enable the "fix_modify"_fix_modify.html {energy} option for
that fix.  The doc pages for individual "fix"_fix.html commands
specify if this should be done.

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[Restrictions:]

Features that are not yet implemented are listed here, in case someone
knows how they could be coded:

It is an error to use "fix shake"_fix_shake.html with minimization
because it turns off bonds that should be included in the potential
energy of the system.  The effect of a fix shake can be approximated
during a minimization by using stiff spring constants for the bonds
and/or angles that would normally be constrained by the SHAKE
algorithm.

"Fix rigid"_fix_rigid.html is also not supported by minimization.  It
is not an error to have it defined, but the energy minimization will
not keep the defined body(s) rigid during the minimization.  Note that
if bonds, angles, etc internal to a rigid body have been turned off
(e.g. via "neigh_modify exclude"_neigh_modify.html), they will not
contribute to the potential energy which is probably not what is
desired.

The volume of the simulation domain is not allowed to change during a
minimization.  Ideally we would allow a fix such as {npt} to impose an
external pressure that would be included in the minimization
(i.e. allow the box dimensions to change), but this has not yet been
implemented.

Pair potentials that produce torque on a particle (e.g. "granular
potentials"_pair_gran.html or the "GayBerne
potential"_pair_gayberne.html for ellipsoidal particles) are not
relaxed by a minimization.  More specifically, radial relaxations are
induced, but no rotations are induced by a minimization, so such a
system will not fully relax.

[Related commands:]

"min_modify"_min_modify.html, "min_style"_min_style.html,
"run_style"_run_style.html

[Default:] none