Find an isocontour by searching along either the x, y or z direction.

As discussed in the part of the manual on Analysis PLUMED contains a number of tools that allow you to calculate a function on a grid. The function on this grid might be a HISTOGRAM as a function of a few collective variables or it might be a phase field that has been calcualted using MULTICOLVARDENS. If this function has one or two input arguments it is relatively straightforward to plot the function. If by contrast the data has a three dimensions it can be difficult to visualize.

This action provides one tool for visualizing these functions. It can be used to search for a set of points on a contour wher the function takes a particular value. In other words, for the function \(f(x,y,z)\) this action would find a set of points \(\{x_c,y_c,z_c\}\) that have:

\[ f(x_c,y_c,z_c) - c = 0 \]

where \(c\) is some constant value that is specified by the user. The points on this contour are find by searching along lines that run parallel to the \(x\), \(y\) or \(z\) axis of the simulation cell. The result is, therefore, a two dimensional function evaluated on a grid that gives us the height of the interface as a function of two coordinates.

It is important to note that this action can only be used to detect countours in three dimensional functions. In addition, this action will fail to find the full set of contour points if the contour does not have the same topology as an infinite plane. If you are uncertain that the isocontours in your function have the appropriate topology you should use FIND_CONTOUR in place of FIND_CONTOUR_SURFACE.

Compulsory keywords
STRIDE ( default=1 ) the frequency with which the data should be collected and added to the quantity being averaged
CLEAR ( default=0 ) the frequency with which to clear all the accumulated data. The default value of 0 implies that all the data will be used and that the grid will never be cleared
GRID the action that creates the input grid you would like to use
CONTOUR the value we would like to draw the contour at in the space
SEARCHDIR In which directions do you wish to search for the contour.
BUFFER ( default=0 ) number of buffer grid points around location where grid was found on last step. If this is zero the full grid is calculated on each step
SERIAL ( default=off ) do the calculation in serial. Do not parallelize
LOWMEM ( default=off ) lower the memory requirements
TIMINGS ( default=off ) output information on the timings of the various parts of the calculation

( default=off ) output the unaveraged quantity/quantities.

LOGWEIGHTS list of actions that calculates log weights that should be used to weight configurations when calculating averages

if your input is a vector field use this to specifiy the component of the input vector field for which you wish to use


The input shown below was used to analyse the results from a simulation of an interface between solid and molten Lennard Jones. The interface between the solid and the liquid was set up in the plane perpendicular to the \(z\) direction of the simulation cell. The input below calculates something akin to a Willard-Chandler dividing surface [70] between the solid phase and the liquid phase. There are two of these interfaces within the simulation box because of the periodic boundary conditions but we were able to determine that one of these two surfaces lies in a particular part of the simulation box. The input below detects the height profile of one of these two interfaces. It does so by computing a phase field average of the FCCUBIC symmetry function using the MULTICOLVARDENS action. Notice that we use the fact that we know roughly where the interface is when specifying how this phase field is to be calculated and specify the region over the \(z\)-axis in which we are going to search for the phase field in the line defining the MULTICOLVARDENS. Once we have calculated the phase field we search for contour points on the lines that run parallel to the \(z\)-direction of the cell box using the FIND_CONTOUR_SURFACE command. The final result is a \(14 \times 14\) grid of values for the height of the interface as a function of the \((x,y)\) position. This grid is then output to a file called contour2.dat.

Notice that the commands below calculate the instantaneous position of the surface separating the solid and liquid and that as such the accumulated average is cleared on every step.

  SPECIES=1-96000 SWITCH={CUBIC D_0=1.2 D_MAX=1.5}
  ALPHA=27 PHI=0.0 THETA=-1.5708 PSI=-2.35619 LABEL=fcc


DUMPGRID GRID=ss2 FILE=contour2.dat FMT=%8.4f STRIDE=1