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### min_modify command

Syntax:

    min_modify keyword values ...

- one or more keyword/value pairs may be listed

      keyword = dmax or line
        dmax value = max
          max = maximum distance for line search to move (distance units)
        line value = backtrack or quadraticor forcezero
          backtrack,quadratic,forcezero = style of linesearch to use

Examples:

    min_modify dmax 0.2

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 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.

By contrast, the quadratic line search algorithm tries to reduce the forces to zero, while guaranteeing that the energy changes is not positive (uphill). For some systems, it may also be more efficient than the backtracking algorithm by requiring fewer energy/force evaluations. The forcezero line search algorithm is similar to quadratic. It may be more efficient than quadratic on some systems.

Restrictions: none

Related commands:

min_style, minimize

Default:

The option defaults are dmax = 0.1 and line = backtrack.