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--- potfiles:format3 [2018/09/24 17:59] – daniel
+++ potfiles:format3 [2018/09/25 17:37] – daniel
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+~~NOTOC~~
 ====== Potential File Format 3  ====
@@ Line 7: / Line 8: @@
 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.
-In a potential file in format 3, each data column has its own range and spacing. Sampling points are
+Potential files using format 3 need to specify one distance block at the beginning and then one potential table for each potential function.
-equidistant in the atomic distance r or (when using EAM-type interactions) electron density n. If we
-have n data columns, the file starts with n lines of the form
-<code>x_begin x_end steps</code>
+== Distance block ==
-specifying the start, end, and the number of function values in each data set. n data columns. The
+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.
-argument x is either the atom distance, or the host electron density. These n lines are then followed
-by n blocks with the function values, one for each atom type pair or atom type. The spacing of function
-values is computed from the begin, end, and number of function values of the corresponding table and
-can be different for the different data columns.
-If the ''#G'' header line is specified, each data block must be preceded by two values specifying the
+Here is an example for a pair potential for two atom types, which requires three potential functions:
+<code>
+## last header line
+#E
+.5 7 10
+.3 6.5 12
+.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.
-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.
 ----