11.2. Error and warning details

Many errors or warnings are self-explanatory and thus straightforward to resolve. However, there are also cases, where there is no single cause and explanation, where LAMMPS can only detect symptoms of an error but not the exact cause, or where the explanation needs to be more detailed than what can be fit into a message printed by the program. The following are discussions of such cases.

11.2.1. Unknown identifier in data file

This error happens when LAMMPS encounters a line of text with an unexpected keyword while reading a data file. This would be either header keywords or section header keywords. This is most commonly due to a mistyped keyword or due to a keyword that is inconsistent with the atom style used.

The header section informs LAMMPS how many entries or lines are expected in the various sections (like Atoms, Masses, Pair Coeffs, etc.) of the data file. If there is a mismatch, LAMMPS will either keep reading beyond the end of a section or stop reading before the section has ended. In that case the next line will not contain a recognized keyword.

Such a mismatch can also happen when the first line of the data is not a comment as required by the format, but a line with a valid header keyword. That would result in LAMMPS expecting, for instance, 0 atoms because the “atoms” header line is the first line and thus treated as a comment.

Another possibility to trigger this error is to have a keyword in the data file that corresponds to a fix (e.g. fix cmap) but the read_data command is missing the (optional) arguments that identify the fix and the header keyword and section keyword or those arguments are inconsistent with the keywords in the data file.

11.2.2. Incorrect format in … section of data file

This error happens when LAMMPS reads the contents of a section of a data file and the number of parameters in the line differs from what is expected. This most commonly happens, when the atom style is different from what is expected for a specific data file since changing the atom style usually changes the format of the line.

This error can also happen when the number of entries indicated in the header of a data file (e.g. the number of atoms) is larger than the number of lines provided (e.g. in the corresponding Atoms section) and then LAMMPS will continue reading into the next section and that would have a completely different format.