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~~NOTOC~~
====== Potential File Format 3  ====

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===== Format specification =====

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

Potential files using format 3 need to specify one distance block at the beginning and then one potential table for each potential function.

== Header ==

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

== Distance block ==

The distance block needs to be specified directly after the header. It needs to contain exactly as many lines as there are potential functions. Each line requires three values, the minimal distance, the cutoff distance for each potential function table and the number of sampling points.

Here is an example for a pair potential for two atom types, which requires three potential functions:
<code>
## last header line
#E

2.5 7 10
2.3 6.5 12
2.4 7 10
</code>

The first line ''2.5 7 10'' specifies that the first potential function starts at $r=2.5$ and has a cutoff distance of $r=7$. The value of $10$ sampling points results in a distance of $\Delta r = 0.5$ between sampling points $((7-2.5) / (10 - 1) = 0.5$)). The potential values are specified at distances $r = 2.5 + n \cdot 0.5, n = 0, \ldots, 9$.

== Function tables ==

Each function table is just a list of function values at the sampling points defined in the distance block.

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.

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===== Examples =====

A sample format 3 potential file for a binary system for an EAM potential is available 
{{:format_3g.pot.txt|with}} or {{:format_3.pot.txt|without}} 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.
<code>
## This is the header
## F -- this is the Format line
## 3 -- this is the Format number
## 1 -- this is the number of potentials in this file
#F 3 1
## E -- this is the End of the header
#E

## Distance block
## There needs to be one line with three numbers for each potential (1 in this example)
## The three numbers are: x_begin x_end num_points
## x_begin -- this is the distance at which the potential starts
## x_end -- this is the distance at which the potential ends (cutoff distance)
## num_points -- this is the number of equidistant sampling points
2 5 7

## Function tables
## One function table for each potential (1 in this example)
## Sampling points are calculated from the distance block
## r = 2
-4.0781157617415625e-01
## r = 2.5
-4.2751242289970970e-01
## r = 3
-2.6989299155853785e-01
## r = 3.5
-1.3013380101538899e-01
## r = 4
-5.7608959288625167e-02
## r = 4.5
-2.8884381992646008e-02
## r = 5
-4.5125648713075998e-02
</code>