All methods in the BigDFT package (gemoetry optimizers in bigdft, global, mhgps,...) can also be used with force fields. The force fields are intended for testing purposes only, as most algorithms are not optimized for the fast force fields (for example, lots of output to screen and disc).
Force fields are activated in the 'mode' section of input.yaml:
mode: method: <Name of force field> mm_paramset: <name of (hard coded) parameter set> mm_paramfile: <name of external parameter file>
In any case, the name of the force field has to be given. If a name of a hard coded parameter set or an external parameter file is required depends on the force field.
Except the Lennard Jones force field, all potentials output atomic units. The Lennard Jones potential uses LJ units and therefore coordinate files have to be given in atomic units (in order to avoid conversion to other units by the file reading routines). For all other force fields, the structures can also be given in Angstroems.
Attention: Don't use periodic potentials for slab or free BC calculations, as some algorithms depend on the degrees of freedom of the system which are determied by the boundary conditions (BC) keyword in the coordinate input file(s).
References for the used parameter sets can be found in the source code and in the standard output.
For some force fields a documentation is not yet available (labeled with 'to be done' (TBD)). However, the usage should become clear from a quick look into the specific module file in the source code.
Available Force Fields
method name: lj hard coded parameter sets: default external parameter file: not supported
mode: method: lj
Attention: The unit in the coordinate files must be specified as bohr/bohrd0 or atomic/atomicd0.
Lenosky Silicon FF for bulk
Lenosky Silicon FF for free BC
A wrapper of the periodic Lenosky potential. Box sizes have to be given in an external file, specified by the mm_paramfile.
method name: lensic hard coded parameter sets: not supported external parameter file: specify a path to the file under the keyword mm_paramfile. Has to contain the box dimensions.
mode: method: lensic mm_paramfile: lensic_cell
where the file lensic_cell contains the dimension of the simulation box:
50 50 50
Units in the parameter file must correspond to the specified in the coordinate file.
Born-Mayer-Huggins-Tosi-Fumi potential for free BC.
method name: bmhtf hard coded parameter sets: NaCl, KCl, LiCl, NaF external parameter file: not supported
mode: method: bmhtf mm_paramset: NaCl
Tersoff potential for periodic BC.
method name: tersoff hard coded parameter sets: C and Si, however, they cannot be specified by the mm_paramset keyword, but are automatically chosen by virtue of the atom names given in the coordinate file. external parameter file: not supported
mode: method: tersoff