The LamellarOrderExternal interaction implements an external potential that was designed to induced layering in block copolymer liquid. The potential energy is a periodic function of the coordinate x along one of the three direction orthogonal directions of an orthorhombic. The function $V_{i}(x)$ for atoms of type $i$ is taken to be of the form

$V_{i}(x) = \lambda A_{i} \tanh \left ( \frac{1}{WqL} \cos( qx ) \right)$

with

$q = \frac{2\pi}{L}n$

where $L$ is the length of the box along the chosen direction, $n$ is the number of periods (i.e., layers) per unit cell, $W$ is a dimensionless interfacial width, $A_{i}$ is a prefactor that can have different values for different atom types, and $\lambda$ is an overall prefactor.

In the parameter field the parameter $A_{i}$ is denoted by an array variable named "prefactor", the overall prefactor $\lambda$ is given by a parameter "externalParameter", the width parameter $W$ is named "interfaceWidth", the integer n is named "periodicity" and the Cartesian axis index is is given by an integer "perpDirection" which can be 0, 1, or 2.

The parameter file format for LamellarOrderExternal is

perpDirection       int
prefactor           Array<float>
externalParameter   float
interfaceWidth      float
periodicity         int

where prefactor is an array in column form. For example, an input file for a system with two atom types (e.g., a diblock copolymer) to induce four layers along the x (index 0) direction might look like:

perpDirection         0
prefactor            -1.0
                      1.0
externalParameter     0.2
interfaceWidth        0.1
periodicity           4