ReaxFF as an AMS engine¶

The ReaxFF AMS engine enables efficient parallel calculations using this empirical reactive molecular mechanics potential. The engine shares all core routines with the standalone ReaxFF program, guaranteeing identical energies and forces between these two codes.

The ReaxFF engine currently supports only molecular (free boundary) and 3D-periodic systems. Non-orthorhombic lattices are supported in an arbitrary orientation. However, the engine is slightly more computationally efficient when the cell is oriented according to the convention used in standalone ReaxFF, i.e. lattice vector c aligned with the z axis and vector b in the yz plane (zero x component). The Lattice block in the system definition then looks like this:

System
   Lattice
     xx xy xz
      0 yy yz
      0  0 zz
   End
End

The only one input key required by the engine is ForceField, used to select the force field file. Force fields included in the Amsterdam Modeling Suite can be easily accessed using their file name, such as CHO.ff.

ForceField

Type:	String
Description:	Path to the force field parameter file. Absolute path if starting with / or ./, relative to $ADFRESOURCES/ForceFields/ReaxFF otherwise.

The Torsions key can be used to enable the improved torsion angle potential. Although the original ReaxFF torsion potential is the default to preserve backward compatibility, the 2013 potential eliminates problematic energy discontinuities and works well with existing force fields.

Torsions

Type:	Multiple Choice
Default value:	Original
Options:	[Original, 2013]
Description:	Version of the torsion potential expression.

The engine can also be switched to a special non-reactive mode useful mainly for initial preparation of molecular dynamics simulations. This mode greatly reduces the occurence of unwanted reactions when starting from an unrelaxed geometry. In these situations, we recommend running a short simulation with the NonReactive key to relieve the initial conformational strain and then restarting the MD run without this key.

Note that if you want to resume or extend an interrupted NonReactive run, it is recommended to also use the EngineRestart AMS key to supply the last ReaxFF .rkf file from the previous run. This enables the engine to load the bonding topology used during the previous run and ensure that the simulation is seamlessly restarted. If the EngineRestart key is not used, bonds will be re-detected in the first step and then preserved during the rest of the simulation.

NonReactive

Type:	Bool
Default value:	False
Description:	Enable the non-reactive mode. Bonds are determined only once at the beginning and subsequent steps only update their bond orders. Thus, no new bonds can form during the simulation, but existing bonds can still stretch and dissociate.

Finally, details of the charge equilibration procedure can be adjusted using the Charges block.

Charges
   Converge
      Charge float
   End
   Predictor
      Method [None | Simple]
   End
   Solver [Direct | CG | MINRESQLP | SparseCG | None]
End

Charges

Type:	Block
Description:	Settings for the polarizable charge model.

Converge

Type:	Block
Description:	Controls the convergence criteria for charge equilibration.

Charge

Type:	Float
Default value:	1e-06
Description:	Requested upper bound on the sum of squared charge residuals.

Predictor

Type:	Block
Description:	Settings for the prediction of new charges before running the solver.

Method

Type:	Multiple Choice
Default value:	Simple
Options:	[None, Simple]
Description:	Method used to predict the charges.

Solver

Type:	Multiple Choice
Default value:	SparseCG
Options:	[Direct, CG, MINRESQLP, SparseCG, None]
Description:	Algorithm used to solve the charge equilibration equations.