Tertiary structure format Textual format Topology format PSF 1.22 The high similarity in the functional form of the two potential energy functions used by AMBER and CHARMM force-fields gives rise to the possible use of one force-field within the other MD engine. Therefore, the conversion of PSF files to AMBER Prmtop format is possible with the use of AMBER chamber (CHARMM - AMBER) program. X-Plor Protein Structure Files (PSF) are structure topology files used by NAMD and CHARMM molecular simulations programs. PSF files contain six main sections of interest: atoms, bonds, angles, dihedrals, improper dihedrals (force terms used to maintain planarity) and cross-terms.