## model.write_psf() — write molecular topology to PSF file

`write_psf(file, xplor=True)`

> This command writes the current model topology (which must have already been created via model.generate_topology() or complete_pdb) to a CHARMM or X-PLOR format PSF file or file handle (see modfile.File()).

> PSF files contain information on all atoms in the model and their types, plus all the connectivity (bonds, angles, etc). By default X-PLOR format PSF files are written, in which each atom has a defined type name. These are more flexible than CHARMM format PSF files, in which each atom type is specified numerically.

> PSF files are generally required in combination with binary format trajectory files, as written by actions.charmm_trajectory().

Example: See actions.charmm_trajectory() command.