This plugin reads electron density maps written in Fsfour format, used by XtalView. The format is briefly described at

Feature-specific notes:

This plugin is used only to read files. VMD does not currently support writing this file type.

Fsfour files are created using the native byte-order of the machine used to create them. This plugin can read maps created with either byte-order.

Symmetry Records
Fsfour maps may contain a collection of records that describe the symmetry of the crystal. Since VMD does not make use of such information currently, these records are ignored.

Unit Cell Geometry
Old versions of the cns2fsfour and ccp2fsfour utilities produced files that lacked the unit cell information that was not used by XTalView. This plugin can currently load these files, but the unit cell will almost certainly be incorrect. Support for such files may become deprecated, users are encouraged to upgrade the the newest versions of these tools.

Unit Cell Origin
Fsfour maps assume a periodic unit cell; therefore, each file contains data for a unit cell beginning at the point <0, 0, 0>.