# Eam/fs file format

Does anyone can confirm the sequence of the ρ cross terms to use in the eam/fs file format.
Lammps documentation reports “density function ρ1β(r) for element β at element 1” (i.e., sequence of columns for ρ1β(r) ρ2β(r) ρ3β(r) ) however old documentation seamed to suggest the opposite order ρβ1(r) …
Besides is ρ1β(r) defined as: ρij(r) = (cij/cii)^2 * (aij/r)^mij?
Thanks for any feedback.

The documentation for the EAM pair styles was last revised in this commit: Clarified order of rho_alpha_beta density functions · lammps/[email protected] · GitHub

For details not mentioned in the documentation page, please consult the corresponding publications.
If you want 100% confirmation, you probably will have to study the source code.

Perhaps @athomps, who revised the EAM docs last, has additional and more specific comments.

The previous version of the code did not specify any order. The change I made in 2020 specifies that the slower index is \beta, the element of the neighbor atom. I examined the relevant lines of code in pair_eam_fs.cpp and pair_eam.cpp, and confirmed that this is correct, with 99% confidence. If you want 100%, you will have to add some print statements to PairEAM::compute().

Regarding the question “ρij(r) = (cij/cii)^2 * (aij/r)^mij?”, that depends on how your *.eam.fs file was generated, and so we can not help you.

This is indeed what I understood from the updated version of the documentation.
I will have some further check on the definition on my side for the ρij(r), but it should be as in yours.
Thanks to both akohlmey and athomps for the quick and clear answers.
Best:)