NOMAD Data Model

NOMAD organizes data into sections, where each section can contain data and additional subsections. This hierarchical organization allows users to browse complex data similarly to browsing files and directories on a computer. Each section follows a specific definition, ensuring that all contained data and subsections have precise names, descriptions, types, shapes, and units. This structured approach not only makes it easier for users themselves to explore the data but also keeps the data and metadata consistent and interoperable, i.e., machine-readable.

To define the structures for data files, NOMAD adopts a data model based on an entity-activity framework. Essentially, each section that is intended to become an entry in NOMAD is a subsection of one of the two main base sections: entities or activities. Entities are objects that persist over time, such as material systems, collections, or instruments. Activities are actions involving entities, including processes, measurements, analyses, and experiments.

Sections in NOMAD are interrelated through inheritance and composition. Inheritance allows sections to inherit properties, attributes, and functions from other sections, building on predefined structures. Composition enables sections to include other sections as subsections, organizing complex data hierarchically. This flexible design makes it possible to incorporate new data types by extending and reusing existing sections.

What are Base Sections?

Base sections are reusable building blocks in NOMAD's data model. The top-most parent base section, which is called BaseSection, provides a set of four global quantities for basic information about entries:

• name: A short human readable and descriptive name.

• datetime: The date and time associated with this section.

• lab_id: An ID string that is unique at least for the lab that produced this data.

• description: Any information that cannot be captured in the other fields.

Both the Entity and Activity base sections, inherit these four global quantities from BaseSection.

What are Entities?

Entities are objects that persist over time while maintaining their identity. In NOMAD, the Entity base section is composed of the three sections System, Collection, and Instrument.

• System: Represents any material system, from atomic to device scale. It includes properties like elemental composition, which tracks the elements and their atomic fractions within the system. CompositeSystem and PureSubstance are two sections that inherit from the System base section.
• Collection: Used to group entities together, such as a batch of substrates.
• Instrument: Describes tools used for material processing or characterization.

What are Activities?

Activities are actions that occur over time and depend on some entity. The Activity section is subclassed by Process, Measurement, Analysis, and Experiment sections. These subclasses are intended to cover all types of activities and should be used instead of inheriting directly from Activity base section.

• Process: Represents a planned process that results in physical changes to a specific material, such as sample preparation or material transformation.
• Measurement: A planned process aimed at producing information about the material entity.
• Analysis: Involves data processing and analysis, producing output data from input data.
• Experiment: Groups activities together, useful for organizing multiple samples or processes under a single experimental framework.

Please note, all sections mentioned here are base sections that inherit from either Entity or Activity, which are themselves subclasses of BaseSection.

For further details, you may refer to the NOMAD's documentation page on How to use base sections. The built-in NOMAD module datamodel.metainfo.basesections contains a set of base sections based on the entity-activity model. There are more base sections available for reuse that are not listed in this module.