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potfiles:format4

# Potential File Format 4

## Format specification

The potential formats 3 and 4 are used for tabulated potentials. Format 4 allows arbitrary sampling points for the potential function tables and supports all header lines.

As with potential files in format 3 each data column has its own spacing. But with potential format 4 the requirement that the sampling points be equidistant is dropped. Arbitrarily spaced sampling points are possible, but the sampling points have to be given in ascending order.

All potential files need a header block at the beginning, which is described in detail here: Potential files.

##### Sampling points block

Preceding the data columns is a block with one line per potential, containing the number of sampling points in the corresponding potential.

Here is an example for a pair potential for two atom types, which requires three potential functions:

## last header line
#E

15
15
15
##### Function tables

Seperated by blank lines then the data columns follow. Each of the tables has the form

r_0     f(r_0)
r_1     f(r_1)
...
r_steps f(r_steps)

Please note that $r_0 < r_1 < \ldots < r_{steps}$ is required. The order of the columns is the same as with format 3.

Similarly as for format 3, for potential functions that are a function of distance (e.g. pair potentials, EAM transfer functions), a smooth cutoff at the final sampling value is assumed and the value is not varied.

If the #G header line is specified, each function table must be preceded by two values specifying the gradient of the function at the lower and upper boundaries. A value of $10^{30}$ or greater triggers the usage of natural splines (curvature 0 at boundaries). You should not make the gradient variable at boundaries where natural splines are specified. This will lead to undesired results.

## Examples

A sample format 4 potential file for a binary system (EAM potential) is available here (with the #G header line).

And here is a very simple pair potential for a monoatomic system. All lines starting with ## are comments and can be removed.

## This is the header
## F -- this is the Format line
## 4 -- this is the Format number
## 1 -- this is the number of potentials in this file
#F 4 1
## E -- this is the End of the header
#E

## Sampling points block
## There needs to be one line with the number of sampling points for each potential (1 in this example)
14

## Function tables
## One function table for each potential (1 in this example)
## The number of lines must be equal to the number provided in the respective line in the distance block (14 in this example)
## Each line has two values, the distance and the potential value
## The first and the last line define the range of the potential
2.7785714285714285e+00 8.4255614529302880e-01
3.2571428571428571e+00 2.0271471509245159e-01
3.7357142857142858e+00 2.2155522312087100e-02
4.2142857142857144e+00 -6.8303638148139376e-02
4.6928571428571431e+00 -4.9043278969706922e-02
5.1714285714285717e+00 -1.0946697911942214e-02
5.6500000000000004e+00 -1.0298525723152156e-02
6.1285714285714290e+00 -6.5685589306463460e-03
6.6071428571428577e+00 -2.0283626156443295e-03
7.0857142857142854e+00 1.2197714420628298e-02
7.5642857142857141e+00 2.2148801387662245e-02
8.0428571428571427e+00 1.4461939319528331e-02
8.5214285714285722e+00 9.9020972820913523e-03
9.0000000000000000e+00 0.0000000000000000e+00
potfiles/format4.txt · Last modified: 2020/10/12 10:50 by pbro