min_modify command

Syntax

min_modify keyword values ...

• one or more keyword/value pairs may be listed

  keyword = dmax or line or norm or alpha_damp or discrete_factor or integrator or tmax
    dmax value = max
      max = maximum distance for line search to move (distance units)
    line value = backtrack or quadratic or forcezero or spin_cubic or spin_none
      backtrack,quadratic,forcezero,spin_cubic,spin_none = style of linesearch to use
    norm value = two or inf or max
      two = Euclidean two-norm (length of 3N vector)
      inf = max force component across all 3-vectors
      max = max force norm across all 3-vectors
    alpha_damp value = damping
      damping = fictitious Gilbert damping for spin minimization (adim)
    discrete_factor value = factor
      factor = discretization factor for adaptive spin timestep (adim)
    integrator value = eulerimplicit or verlet
      time integration scheme for fire minimization
    tmax value = factor
      factor = maximum adaptive timestep for fire minimization (adim)

Examples

min_modify dmax 0.2
min_modify integrator verlet tmax 4

Description

This command sets parameters that affect the energy minimization algorithms selected by the min_style command. The various settings may affect the convergence rate and overall number of force evaluations required by a minimization, so users can experiment with these parameters to tune their minimizations.

The cg and sd minimization styles have an outer iteration and an inner iteration which is steps along a one-dimensional line search in a particular search direction. The dmax parameter is how far any atom can move in a single line search in any dimension (x, y, or z). For the quickmin and fire minimization styles, the dmax setting is how far any atom can move in a single iteration (timestep). Thus a value of 0.1 in real units means no atom will move further than 0.1 Angstroms in a single outer iteration. This prevents highly overlapped atoms from being moved long distances (e.g. through another atom) due to large forces.

The choice of line search algorithm for the cg and sd minimization styles can be selected via the line keyword. The default quadratic line search algorithm starts out using the robust backtracking method described below. However, once the system gets close to a local minimum and the linesearch steps get small, so that the energy is approximately quadratic in the step length, it uses the estimated location of zero gradient as the linesearch step, provided the energy change is downhill. This becomes more efficient than backtracking for highly-converged relaxations. The forcezero line search algorithm is similar to quadratic. It may be more efficient than quadratic on some systems.

The backtracking search is robust and should always find a local energy minimum. However, it will “converge” when it can no longer reduce the energy of the system. Individual atom forces may still be larger than desired at this point, because the energy change is measured as the difference of two large values (energy before and energy after) and that difference may be smaller than machine epsilon even if atoms could move in the gradient direction to reduce forces further.

The choice of a norm can be modified for the min styles cg, sd, quickmin, fire, fire/old, spin, spin/cg and spin/lbfgs using the norm keyword. The default two norm computes the 2-norm (Euclidean length) of the global force vector:

\[|| \vec{F} ||_{2} = \sqrt{\vec{F}_1^2+ \cdots + \vec{F}_N^2}\]

The max norm computes the length of the 3-vector force for each atom (2-norm), and takes the maximum value of those across all atoms

\[|| \vec{F} ||_{max} = {\rm max}\left(||\vec{F}_1||, \cdots, ||\vec{F}_N||\right)\]

The inf norm takes the maximum component across the forces of all atoms in the system:

\[|| \vec{F} ||_{inf} = {\rm max}\left(|F_1^1|, |F_1^2|, |F_1^3| \cdots, |F_N^1|, |F_N^2|, |F_N^3|\right)\]

For the min styles spin, spin/cg and spin/lbfgs, the force norm is replaced by the spin-torque norm.

Keywords alpha_damp and discrete_factor only make sense when a min_spin command is declared. Keyword alpha_damp defines an analog of a magnetic Gilbert damping. It defines a relaxation rate toward an equilibrium for a given magnetic system. Keyword discrete_factor defines a discretization factor for the adaptive timestep used in the spin minimization. See min_spin for more information about those quantities.

The choice of a line search algorithm for the spin/cg and spin/lbfgs styles can be specified via the line keyword. The spin_cubic and spin_none keywords only make sense when one of those two minimization styles is declared. The spin_cubic performs the line search based on a cubic interpolation of the energy along the search direction. The spin_none keyword deactivates the line search procedure. The spin_none is a default value for line keyword for both spin/lbfgs and spin/cg. Convergence of spin/lbfgs can be more robust if spin_cubic line search is used.

The Newton integrator used for fire minimization can be selected to be either the symplectic Euler (eulerimplicit) or velocity Verlet (verlet). tmax defines the maximum value for the adaptive timestep during a fire minimization. It is a multiplication factor applied to the current timestep (not in time unit). For example, tmax = 4.0 with a timestep of 2fs, means that the maximum value the timestep can reach during a fire minimization is 4fs. Note that parameter defaults has been chosen to be reliable in most cases, but one should consider adjusting timestep and tmax to optimize the minimization for large or complex systems. Other parameters of the fire minimization can be tuned (tmin, delaystep, dtgrow, dtshrink, alpha0, and alphashrink). Please refer to the references describing the min_style fire. An additional stopping criteria vdfmax is used by fire in order to avoid unnecessary looping when it is reasonable to think the system will not be relaxed further. Note that in this case the system will NOT have reached your minimization criteria. This could happen when the system comes to be stuck in a local basin of the phase space. vdfmax is the maximum number of consecutive iterations with P(t) < 0.

The min_style fire is an optimized implementation of min_style fire/old. It can however behave similarly to the fire/old style by using the following set of parameters:

min_modify integrator eulerexplicit tmax 10.0 tmin 0.0 delaystep 5 &
          dtgrow 1.1 dtshrink 0.5 alpha0 0.1 alphashrink 0.99 &
          vdfmax 100000 halfstepback no initialdelay no

Restrictions

For magnetic GNEB calculations, only spin_none value for line keyword can be used when minimization styles spin/cg and spin/lbfgs are employed. See neb/spin for more explanation.

Related commands

min_style, minimize

Default

The option defaults are dmax = 0.1, line = quadratic and norm = two.

For the spin, spin/cg and spin/lbfgs styles, the option defaults are alpha_damp = 1.0, discrete_factor = 10.0, line = spin_none, and norm = euclidean.

For the fire style, the option defaults are integrator = eulerimplicit, tmax = 10.0, tmin = 0.02, delaystep = 20, dtgrow = 1.1, dtshrink = 0.5, alpha0 = 0.25, alphashrink = 0.99, vdfmax = 2000, halfstepback = yes and initialdelay = yes.