# Particles¶

A system of N particles is represented in OVITO as a group of particle properties, each being a uniform data array of length N:

The property array with the name Position is always part of the particle dataset and contains the Cartesian coordinates of the particles. All other properties are optional. Whether they are present or not depends on the kind of simulation file you import and the modifiers you apply to the dataset within OVITO. That’s because modifiers may add new particle properties to the set, e.g., to store the results of a computation they perform.

You can open the data inspector panel of OVITO to view all particle properties that currently exist in the output of the data pipeline.

## Typed properties¶

A typed particle property is a property array containing discrete numeric values and a supplementary mapping of these numeric values to corresponding type definitions. The Particle Type property is a typical example for such a typed property. It stores each particle’s chemical type encoded as a unique integer value (1, 2, 3, …), the so-called numeric type identifier. Additionally, the Particle Type property stores a list of records defining the types, which establishes a mapping between the numeric type ID of each particle and the auxiliary information associated with that type, e.g. its name and display color:

Note that a dataset may not just contain a single typed property like the Particle Type property. In fact, several typed properties can exist simultaneously, establishing several orthogonal classifications. Examples are the particle properties Residue Type, Structure Type, and Molecule Type.

All typed properties read from an imported simulation file are accessible in the pipeline editor as shown in the screenshot on the right. Here you can edit each type’s attributes. In case of the Particle Type property, these settings directly affect how OVITO renders the particles belonging to the type.

Particle types named after one of the standard chemical elements get automatically initialized with appropriate default values for the display color, display radius, van der Waals radius, and mass. If necessary, you can change the default values permanently for each type using the corresponding presets menus indicated in the screenshot. You can even specify default parameters for particle types having generic names such as “Type 1”, “Type 2”, etc., which may be necessary if the imported simulation file contains numeric type information but no type names.