style = cg or hftn or sd or quickmin or fire or fire/old or spin or spin/cg or spin/lbfgs
min_style cg min_style spin min_style fire
Choose a minimization algorithm to use when a minimize command is performed.
Style cg is the Polak-Ribiere version of the conjugate gradient (CG) algorithm. At each iteration the force gradient is combined with the previous iteration information to compute a new search direction perpendicular (conjugate) to the previous search direction. The PR variant affects how the direction is chosen and how the CG method is restarted when it ceases to make progress. The PR variant is thought to be the most effective CG choice for most problems.
Style hftn is a Hessian-free truncated Newton algorithm. At each iteration a quadratic model of the energy potential is solved by a conjugate gradient inner iteration. The Hessian (second derivatives) of the energy is not formed directly, but approximated in each conjugate search direction by a finite difference directional derivative. When close to an energy minimum, the algorithm behaves like a Newton method and exhibits a quadratic convergence rate to high accuracy. In most cases the behavior of hftn is similar to cg, but it offers an alternative if cg seems to perform poorly. This style is not affected by the min_modify command.
Style sd is a steepest descent algorithm. At each iteration, the search direction is set to the downhill direction corresponding to the force vector (negative gradient of energy). Typically, steepest descent will not converge as quickly as CG, but may be more robust in some situations.
Style quickmin is a damped dynamics method described in (Sheppard), where the damping parameter is related to the projection of the velocity vector along the current force vector for each atom. The velocity of each atom is initialized to 0.0 by this style, at the beginning of a minimization.
Style fire is a damped dynamics method described in (Bitzek), which is similar to quickmin but adds a variable timestep and alters the projection operation to maintain components of the velocity non-parallel to the current force vector. The velocity of each atom is initialized to 0.0 by this style, at the beginning of a minimization. This style correspond to an optimized version described in (Guenole) that include different time integration schemes and defaults parameters. The default parameters can be modified with the command min_modify.
Style fire/old is the original implementation of fire in Lammps, conserved for backward compatibility. The main differences regarding the current version fire are: time integration by Explicit Euler only, different sequence in maintaining velocity components non-parallel to the current force vector and hard-coded minimization parameters. A complete description of the differences between fire/old and fire can be found in (Guenole) (where the current fire in LAMMPS is called fire2.0). By using an appropriate set of parameters, fire can behave similar to fire/old, as described in the min_modify command.
Style spin is a damped spin dynamics with an adaptive timestep.
Style spin/cg uses an orthogonal spin optimization (OSO) combined to a conjugate gradient (CG) approach to minimize spin configurations.
Style spin/lbfgs uses an orthogonal spin optimization (OSO) combined to a limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) approach to minimize spin configurations.
See the min/spin doc page for more information about the spin, spin/cg and spin/lbfgs styles.
Either the quickmin, fire and fire/old styles are useful in the context of nudged elastic band (NEB) calculations via the neb command.
Either the spin, spin/cg and spin/lbfgs styles are useful in the context of magnetic geodesic nudged elastic band (GNEB) calculations via the neb/spin command.
The damped dynamic minimizers use whatever timestep you have defined via the timestep command. Often they will converge more quickly if you use a timestep about 10x larger than you would normally use for dynamics simulations. For fire, the default timestep is recommended to be equal to the one you would normally use for dynamics simulations.
Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Speed packages doc page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.
These accelerated styles are part of the GPU, USER-INTEL, KOKKOS, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package doc page for more info.
You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.
See the Speed packages doc page for more instructions on how to use the accelerated styles effectively.
(Sheppard) Sheppard, Terrell, Henkelman, J Chem Phys, 128, 134106 (2008). See ref 1 in this paper for original reference to Qmin in Jonsson, Mills, Jacobsen.
(Bitzek) Bitzek, Koskinen, Gahler, Moseler, Gumbsch, Phys Rev Lett, 97, 170201 (2006).
(Guenole) Guenole, Noehring, Vaid, Houlle, Xie, Prakash, Bitzek, Comput Mater Sci, (2020), in press (arXiv:190802038).