# Properties¶

When reading files, chemfiles read multiple kind of data: positions, velocities, atomic informations (name, type, mass, charge, …), bonds and other connectivity elements. This model allow to read the most commonly available data in various formats. But sometimes, a format defines additional information. Instead of adding a new field/function for every kind of data there can be in a file, chemfiles defines a generic interface to read and store this additional data. These additional data are stored inside properties.

A property has a name and a value. The value can either be a real number, a string, a boolean value (true/false) or a 3 dimmensional vector. A property is either stored inside an atom, and associated with this atom (for example the total atomic force), or stored in and associated with a frame. The later case is used for general properties, such as the temperature of the system, or the author of the file.

This section documents which format set and use properties.

## Atomic properties¶

• is_hetatm (boolean): set and used by the PDB format. On reading, this property is set to true when an atom is defined with a HETATM record, and false when the atom is defined with an ATOM record. On writing, this property is used to determine whether to emit an HETATM or an ATOM record. If the property is not set, HETATM is used.

## Frame properties¶

No property on frame is used by the format reader and writers yet.