The graphical interface of the NOMAD Meta Info displays all common, code-independent, metadata and their relations (see Fig. 1). Here, metadata are organized into separate sub-pages. They are listed in alphabetical order (), and can be filtered through a number of search boxes by their name, parent metadata element, and meta info type ().
The set of information for each metadata is presented in the right panel (). It starts with the metadata name followed by several metadata attributes including the type and description, and a graph diagram showing the relationships between metadata concluded by their relationships (shown below in Fig. 2). Below metadata attributes are explained in more detail.
Each metadata, beside its name, can have up to six additional attributes.
- The name is a label that identifies the metadata. The name is reported on the top left of the panel, above the list of other attributes. It is colored in red if the metadata is derived from other metadata, otherwise it is colored in black.
- The type specifies the category used to classify the metadata. It can have the following values:
Section (): A section groups related quantities. The most important sections are:
section_run: Represents a single “run” of one of the supported electronic-structure programs. What exactly is contained in a run depends on the code and the type of the calculation (e.g., DFT, force field, Quantum Monte Carlo, Tight binding, or Wave-function method).
section_method: Contains parameters that define the level of theory, its approximation, and the numerical convergence parameters.
section_system: Contains all necessary information to describe the simulated physical system, e.g., the atomic configuration, the definition of the unit cell for periodic calculations, the external potentials, and other code-dependent parameters to initiate a computation.
section_single_configuration_calculation: Contains the results for a system as defined in a single section_method and a single section_system.
section_scf_iteration: Represents a self-consistent field iteration and provides detailed information about the used procedure and the computed quantities.
The descendants of these five sections can be filtered with the “Select Parent Section” selection menu (see ).
Abstract (): An abstract metadata classifies data-specific types. It is useful for querying and filtering data by using the “Select Abstract Type” selection menu (see ).
Concrete Value (): A concrete value stores values as specified by the Data Type property.
Dimension (): This type stores the size of a set of metadata, and reveals their relationships. Metadata with such a type can be filtered via “Select type” selection menu (see ).
- A short text in natural language that summarizes the meaning and the expected content of the metadata.
- Data Type:
Data type is a self-descriptive attribute that can have one of the following values:
- f: Floating point number,
- i: Integer number,
- C: Unicode string,
- b: Boolean value,
- D: JSON object, (i.e. a key-value list containing strings, numbers, lists, and other lists),
- r: Integer value representing the index of a referenced metadata.
This attribute defines the size of the metadata. It can take the following values:
-  for a scalar (or a string for Data Type C),
- [ x ] for an array,
- [ x, y ] for a matrix,
- [ x, y, z ] for a third-order tensor,
- … and so on.
The values x, y, z can be numbers or names of metadata of Type Dimension ().
- This attribute specifies the unit of a metadata expressed in terms of the International System of Units (SI), i.e., length (meter); energy (joule); pressure (pascal); force (newton), and so on. It’s only available for metadata of Type Concrete Value ().
- This attribute is only present if the metadata is derived from other metadata. In this case the metadata name is printed in red.
Relations between metadata are visualized by graph diagrams (see Fig. 2). The graph diagrams are interactive: the viewer can be moved and zoomed, and a direct (mouse) click on a node redirects to the definition of the corresponding metadata.
In the graph digram, the current selected metadata is marked by a five-fold star (). The arrows show the relations of metadata as a hierarchical tree with the root element being on the top left. Metadata at the starting point of the arrows are called “children”, and metadata at the end point are called “parents”. There are two type of arrows:
- Azure arrow ():
- This arrow displays “child → parents” relationships. The children are either contained in (Type: Section ) or classified as being of the abstract type (Type: Abstract ) specified by the parent metadata.
- Red arrow ():
- The children of the red arrows are of Type Concrete Value () with Data Type r and references parent metadata of Type Section. Metadata that have such a reference, represent symbolic links and establish connections between different Sections.
Those metadata which are direct children and explicit parents of the current selected metadata are summarized in an ancestor section below the graph diagram. Not all metadata must have such a section, but metadata of Types Section () or Abstract () always have one.