WCA

WCAPotential #

The well-known WCA (Weeks-Chandler-Andersen) potential. It is

\[U(r) = \begin{cases} 4\epsilon \left[\left(\frac{\sigma_{ij}}{r}\right)^{12} - \left(\frac{\sigma_{ij}}{r}\right)^6\right] + \epsilon & (r < \sigma_{ij}\sqrt[6]{2})\\ 0 & (r \geq \sigma_{ij}\sqrt[6]{2})\\ \end{cases}\]

Example #

There are two different way to define the parameters. You can either use the normal combining rule or define all the pair-paremeters.

By defining parameters for each particle, it uses Lorentz-Berthelot combining rule.

\[\sigma_{ij} = \frac{\sigma_i + \sigma_j}{2} \\ \epsilon_{ij} = \sqrt{\epsilon_i\epsilon_j}\]
[[forcefields.global]]
interaction = "Pair"
potential   = "WCA"
ignore.molecule = "Nothing"
ignore.particles_within.bond    = 3
ignore.particles_within.contact = 1
spatial_partition = {type = "CellList", margin = 0.2}

parameters = [
    {index = 0, offset = 100, sigma = 2.0, epsilon = 10.0},
]

To provide pair-parameters manually, define table and give names to the particles.

[[forcefields.global]]
interaction = "Pair"
potential   = "LennardJonesAttractive"
ignore.molecule = "Nothing"
ignore.particles_within.bond    = 3
ignore.particles_within.contact = 1
spatial_partition = {type = "CellList", margin = 0.2}
cutoff = 2.5

table.A.A = {sigma = 1.0, epsilon = 2.0}
table.A.B = {sigma = 3.0, epsilon = 1.0} # B.A will be the same
table.B.B = {sigma = 1.0, epsilon = 1.5}
parameters = [
    {index = 0, offset = 100, name = "A"},
    {index = 1, offset = 100, name = "B"},
    # ...
]

Input Reference #

  • index: Integer
    • The index of the particle.
  • offset: Integer (optional. By default, 0.)
    • Offset value of the index.
  • sigma: Floating
    • It determines the effective particle size. If no table is given, this is required.
  • epsilon: Floating
    • It determines the strength of the potential. If no table is given, this is required.
  • name: String
    • Name of the particle. If table is given, this is required.

Since this potential becomes exactly 0 at \( r = \sigma\sqrt[6]{2} \) , always this cutoff distance is used. You don’t need to set cutoff.

For other values, see Pair.