CHARMM Format
The CHARMM molecular dynamics package can read and write a number of standard chemical and biochemical file formats; however, the CARD (coordinate) and PSF (protein structure file) are largely unique to CHARMM. The CARD format is fixed-column-width, resembles the PDB format, and is used exclusively for storing atomic coordinates. The PSF file contains atomic connectivity information (which describes atomic bonds) and is required before beginning a simulation. The typical file extensions used are .crd and .psf respectively.
Read more about this topic: Chemical File Format