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User Guide

Potential File Tools

The following utility programs deal with IMD potential files:

cutpot truncate a potential file smoothly
imd_mklj compute a Lennard-Jones potential file


Cutpot can be used to shorten pair potentials. For each data column, a cutoff radius (or radius squared) must be specified. Up to this radius, the returned potential is equal to the input potential, but beyond thes radius it is replaced by a function which tends smoothly to zero. The resulting potential is continuous and has a continuous first derivative.

Each line of the input potential starts either with a radius or a squared radius, which is followed by the corresponding potential values. The radii or squared radii, whichever are used, must be equally spaced. For the output potential, only equally spaced, squared radii are used. This format is required by IMD. The cutoff values must be given in the same scale as the input potential, i.e., as radii for linear spacing, and as squared radii for quadratic spacing.

There are two tail functions possible, a quadratic polynomial b(a-x)^2, and an exponential function b*exp(-c/(a-x)). In the latter case, for each column a further parameter, the steepness, can be specified, which controls how fast the potential goes to zero. The default steepness value is 2.0.

Cutpot requires the following parameters (here with example values), which must be given in the parameter file:

  ncols      4                  # number of data (potential) columns
  nsteps     2000               # number of lines in output potential
  infile     input.pot          # input potential file
  outfile    output.pot         # output potential file
  spacing    lin                # spacing of input potential; lin or sqr
  tailtype   sqr                # type of tail function; sqr or exp
  rcut       1.4 1.6 1.6 1.6    # cutoff values, where tail function starts
  steepness  1.5 1.5 1.5 1.5    # steepness of exponential tail function


  gmake cutpot
  cutpot <paramfile>


Computes a Lennard-Jones-potential table for configurations containing two types of atoms. Very simple hack!


  gmake imd_mklj
It prompts for the following input parameters:

Ausgabedatei: Name of potential table file
Radius Anfang: Lowest value of atomic distance
Radius Ende: Largest value of atomic distance
Zahl Schritte: Number of steps in table
Exponent p: Exponent of the repulsive part
Exponent q: Exponent of the attractive part
Energie11: Energy between particle of type 1 and type 1
Energie22: Energy between type 2 and type 2
Energie12: Energy between type 1 and type 2
Sigma11: Nearest-Neighbour-distance between type 1 and type 1
Sigma22: Nearest-Neighbour-distance between type 2 and type 2
Sigma21: Nearest-Neighbour-distance between type 1 and type 2