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Description
When running in a boosted frame, all parameters are entered in the lab frame in the input file, and transformed inside WarpX (for convenience).
This is however not true for external fields on particles (and I believe for external fields on the grid either), where the expression used in the code is directly the one parsed from the input file.
The solution would be to transform them in WarpX, as we do for other quantities. Currently, the only way to use external fields for a boosted-frame simulation is to do as this inputs file, where the Lorentz transform is done directly in the parsed expression, which is inconsistent with the rest of the input file, error-prone and not very readable.