.. index:: min_modify min_modify command ================== Syntax """""" .. code-block:: LAMMPS min_modify keyword values ... * one or more keyword/value pairs may be listed .. parsed-literal:: 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 magnetic 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 """""""" .. code-block:: LAMMPS 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 :doc:`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 :doc:`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: .. math:: || \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 .. math:: || \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: .. math:: || \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 :doc:`min_spin ` command is declared. Keyword *alpha_damp* defines an analog of a magnetic 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 :doc:`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 :doc:`timestep ` (not in time unit). For example, *tmax* = 4.0 with a :doc:`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 :doc:`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 :doc:`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 :doc:`min_style ` *fire* is an optimized implementation of :doc:`min_style ` *fire/old*. It can however behave similarly to the *fire/old* style by using the following set of parameters: .. code-block:: LAMMPS 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 :doc:`neb/spin ` for more explanation. Related commands """""""""""""""" :doc:`min_style `, :doc:`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.