The interactions currently supported by potfit are either defined in terms of tabulated functions or given by analytic parameters. Typical functions are central pair potentials, which depend on the distance of two atoms and the two atom types involved. For tabulated potentials such functions are given with steps which are equidistant in the distance, which is the fundamental difference to IMD, where those functions are tabulated with steps equidistant in the square of the distance. Each potential file consists of a header and a body, which are fundamentally different for tabulated and analytic potentials.

The number of potentials required for a specific interaction is given on the bottom of each interaction page.

The header is comparable to headers in IMD. It 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 depends on the potential model used. The header has the following format:

## this is just a comment line
#F 3 n
#T <TYPE>
#C Mg Zn
#I 0 1 .... n entries (this line is optional)
#G 2 3 .... n entries (this line is optional)
## this is another comment line
#E this is the very last header line

Header lines start with the hash character #. If the second character is F, then the format specifier 0, 3 or 4 (to distinguish from IMD potential files, format types 1 or 2) and the number n of data columns of the 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 C, F, E, T or G are treated as comments, but it is recommended that such lines start with two # characters.

The line starting with #T specifies the type of potential this file contains. This is very helpful if you are dealing with different potentials. <TYPE> can be PAIR, EAM, ADP or ELSTAT.

The line #I defines whether a whole potential function should be kept constant during a force matching run. Each digit corresponds to one function table, so there should be n digits in total. A value of 0 allows potfit to change the corresponding potential function, a value of 1 keeps it fixed. This is useful for example if a third component is to be added to a binary system and only the additional potentials are to be allowed to change.

The line #G specifies whether the gradient of the interpolation splines should be treated as fixed or as an additional degree of freedom. Each digit corresponds to one function table, so there should be n digits in total. The information is bit-coded, 0 declares both lower and upper gradient fixed, 1 allows adjusting the gradient at the upper end, 2 at the lower end, and 3 at both ends. This does not apply to analytic potentials.

The line #C specifies the chemical elements the potentials belong to. Currently this line is not read for potentials, it is only given in the output to facilitate the handling of potential files. In addition there will also be a comment on how the potentials are ordered, for the ternary system AlPdMn a header for an EAM-potential could look like the following:

#F 0 12
#C Al Pd Mn
## Al-Al Al-Pd Al-Mn Pd-Pd Pd-Mn Mn-Mn Al Pd Mn Al Pd Mn
#I 0 0 0 0 0 0 0 0 0 0 0 0
#E

The order of the different potential functions for a specific potential can be found on the corresponding potential page.