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Node classes

There are two Node sub-classes, Data and Calculation. The Code nodes can be considered as sub-classes of Data nodes. JobCalculations, InlineCalulations and the WorkCalculations are subclasses of the Calculation class.

AiIDA graph

The AiIDA graph is a directed graph. The links of the graph are arrows connecting two nodes. An arrow (x, y) is considered to be directed from x to y; y is called the head and x is called the tail of the arrow; y is said to be a direct successor of x and x is said to be a direct predecessor of y. If a path leads from x to y, then y is said to be a successor of x and reachable from x, and x is said to be a predecessor of y. The arrow (y, x) is called the inverted arrow of (x, y).

AiiDA links

The following table summarises the available AiiDA links and their properties. Each line of the table corresponds to a different link, it mentions the tail of the arrow (from column), the head of the arrow (to column) and finally the constraints that are related to the label of the link and the nodes involved.

Link type

Tail of link (from)

Head of link (to)

Unique condition (combinations that should be unique in the link table)
========= =================== ================= ==========================================
INPUT Data Calculation (head Calculation node, link label)
CREATE Calculation Data (head Data node)
RETURN Calculation Data (tail Calculation node, label)
CALL Calculation Calculation (head Calculation node)

There are four different links available: INPUT, CREATE, RETURN and CALL.

  • The INPUT link is always from a Data node (tail of the link) to a Calculation node (head of the link). The unique constraint means that each Calculation node can have only one input Data node with given label (i.e., a calculation cannot accept two inputs with the same label).
  • The CREATE link is always from a Calculation node (tail of the link) to a Data node (head of the link). The unique constraint means that each Data node can be created by one and only one Calculation node.
  • The RETURN link is always from a Calculation node (tail of the link) to a Data node (head of the link). The unique constraint means that a Calculation cannot return two or more Data nodes with the same label. Code implementation detail: For the moment there is always and only a CREATE link from a JobCalculation to the generated Data. A RETURN link is implied with the conditions of a RETURN link (the implementation will be corrected to comply shortly).
  • The CALL link is always from a Calculation to another Calculation node. A given Calculation node cannot be called by more than one Calculation node. In practice, the caller cannot be a JobCalculation but it is always a WorkCalculation. Instead called calculations can be of any subclass of Calculation.

Graph navigation

The links can be followed in both possible directions (forward & reverse) using the QueryBuilder. This requires to define additional “names” for each direction of the link, and they are documented at the QueryBuilder section <../querying/querybuilder/usage>. For example, if there is an INPUT link from data D to calculation C, D is the “input_of” C, or equivalently D is the “output_of” C. Currently, in the QueryBuilder, input_of and output_of refer to any link type, where C is the head of the arrow and D is the tail.