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Potential File Format

Some of the interactions supported by IMD are defined in terms of tabulated functions. Typical such functions are central pair potentials, which depend on the distance of two atoms and the two atom types involved. Such functions have to be tabulated with steps which are equidistant in the square of the distance. For EAM interactions one also needs functions which depend on one atom type and the host electron density at the position of that atom. Such functions are tabulated with steps equidistant in the electron density.

Potential File Header

Tabulated functions can be given in either of two formats. The header specifies in which format the current file is, and how many data columns the tabulated function has for each argument. The number of data columns is either the number of atom type pairs (i.e., the square of the number of atom types), or, in the case of the EAM host electron density, the number of atom types. The header has the following format:

## this is just a comment line
#F f n
## this is another comment line
#E this is the very last header line

Header lines start with the character '#'. If the second character is 'F', then the format specifyer f (either 1 or 2) and the number n of data columns of the tabulated functions are read from the rest of the line. The very last header line must have a character 'E' at the second position. Header lines with a second character different from 'F' or 'E' are treated as comments, but it is recommended that such lines start with two '#' characters.

Format 1

In a potential file in format 1, each line contains first the function argument, either the square of the distance (radius) or the host electron density, and then all data columns, i.e., the function values for this argument for the different atom type pairs or atom types. If the atom types run from 0 to N, the lines in a format 1 file have either of the following formats:

r^2    V_00 V_01 ... V_0N V_10 V_11 ... V_NN 
rho_h  E_0 ... E_N

Note that if a function of the square of the distance given in format 1 does not vanish at the end of the tabulation, it is shifted so that it does vanish. This is for backwards compatibility; it would be better to change the potential file once and for all.

The disadvantage of format 1 is that all data columns have to be tabulated for the same range and with the same spacing.

Format 2

In a potential file in format 2, each data column has its own range and spacing. If we have n data columns, the file starts with n lines of the form

x_begin x_end dx
specifying the start, end, and spacing of the tabulation of the n data columns. The argument x is either the square of 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 number of function values is computed from the begin, end, and spacing values of the corresponding table. They can be different for the different data columns. The order of the different data columns is as described for format 1.

Backwards Compatibility

For backwards compatibility, potential files without header are also accepted, but a warning is printed. A default format is then assumed, which is format 2 for EAM interactions, and format 1 for everything else.