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--- potfiles:format4 [2018/09/22 13:40] – created daniel
+++ potfiles:format4 [2020/10/12 10:50] (current) – [Format specification] #G header line pbro
@@ Line 1: / Line 1: @@
-====  Format 4  ====
+~~NOTOC~~
+====== 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
 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 order.
+points are possible, but the sampling points have to be given in ascending order.
-Preceding the data columns is a header with one line per column, containing the number of sampling
+== Header ==
-points in the corresponding column. Seperated by blank lines then the data columns follow. Each of
-the columns has the form
+All potential files need a header block at the beginning, which is described in detail here: [[potfiles:main|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:
+<code>
+## last header line
+#E
+</code>
+== Function tables ==
+Seperated by blank lines then the data columns follow. Each of the tables has the form
 <code>
 r_0     f(r_0)
@@ Line 21: / Line 46: @@
 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
 {{:format_4g.pot.txt|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.
+<code>
+## 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)
+## 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
+.7785714285714285e+00 8.4255614529302880e-01
+.2571428571428571e+00 2.0271471509245159e-01
+.7357142857142858e+00 2.2155522312087100e-02
+.2142857142857144e+00 -6.8303638148139376e-02
+.6928571428571431e+00 -4.9043278969706922e-02
+.1714285714285717e+00 -1.0946697911942214e-02
+.6500000000000004e+00 -1.0298525723152156e-02
+.1285714285714290e+00 -6.5685589306463460e-03
+.6071428571428577e+00 -2.0283626156443295e-03
+.0857142857142854e+00 1.2197714420628298e-02
+.5642857142857141e+00 2.2148801387662245e-02
+.0428571428571427e+00 1.4461939319528331e-02
+.5214285714285722e+00 9.9020972820913523e-03
+.0000000000000000e+00 0.0000000000000000e+00
+</code>