Planar wall potential

This module implements the external potential for a collection of planar walls.

Position vector {\vec R}_i of the wall i is given by {\hat n_i} \cdot {{\vec R}_i} = r_{0,i} , where {\hat n} = {\vec n} / {|\vec n|} is the normalized outward normal vector to the wall surface. d_i= ({\vec r}_i \cdot \hat{n}_i - r_{0,i}) is the distance of a particle, at position {\vec r}, from the wall i.

U\left(d_i \right) = \varepsilon_i^\alpha \left( \frac{2}{15} \left(\frac{\sigma_i^\alpha}{d_i} \right)^9 - w_i^\alpha \left(\frac{\sigma_i^\alpha}{d_i} \right)^3 \right)

is the interaction energy of a particle of species \alpha due to the wall i. While computing U\left(d_i \right) we always make d_i positive.

The potential is truncated at a cutoff distance r_c and further transformed to a C^2 continuous function using halmd.mdsim.forces.pair_trunc.

The parameters \varepsilon, \sigma, w and r_c depend on both the wall and the species. For example, \varepsilon[i][\alpha] contains \varepsilon_i^\alpha, where indices i, \alpha run over the wall and species respectively.

class halmd.mdsim.potentials.external.planar_wall(args)

Construct the planar wall module.

Parameters:
  • args (table) – keyword arguments.
  • args.offset (table) – positions of the walls r_{0,i} in MD units.
  • args.surface_normal (number) – outward normal vectors to the wall surfaces \vec n in MD units.
  • args.epsilon (matrix) – interaction strengths \varepsilon_i^\alpha in MD units.
  • args.sigma (matrix) – interaction ranges \sigma_i^\alpha in MD units.
  • args.wetting (matrix) – wetting parameters w_i^\alpha in MD units.
  • args.cutoff (matrix) – cutoff lengths r_{c,i}^\alpha in MD units.
  • args.smoothing (number) – smoothing parameter h for the C^2 continuous truncation in MD units.
  • args.memory (string) – select memory location (optional).
  • args.label (string) – instance label (optional).

If all elements of a parameter sequence are equal, a single value may be passed instead. In this case, species must be specified.

If the argument species is omitted, it is inferred from the length of the parameter sequences.

The supported values for memory are “host” and “gpu”. If memory is not specified, the memory location is selected according to the compute device.

offset

Sequence with the wall position r_{0,i}.

surface_normal

Sequence with outward normal vector to the wall surface \vec {n}_i.

epsilon

Sequence with interaction strength \varepsilon_i^\alpha.

sigma

Sequence with interaction range \sigma_i^\alpha.

wetting

Sequence with wetting parameter w_i^\alpha.

cutoff

Sequence with cutoff length r_{c,i}^\alpha.

smoothing

Sequence with smoothing parameter h for the C^2 continuous truncation.

description

Name of potential for profiler.

memory

Device where the particle memory resides.