Add optimization_type property for structures

optimade.rst

@@ -2860,6 +2860,55 @@ structure\_features
 
   - A structure having implicit atoms and using assemblies: :val:`["assemblies", "implicit_atoms"]`
 
+structure\_origin
+~~~~~~~~~~~~~~~~~
+
+- **Description**: A string that describes aspects of the origin of the structural data to indicate if it is based directly or indirectly on experimental evidence, or inferred from other sources, giving some information on whether the structural data was obtained with the intent of representing a structure that can exist in nature or be synthesized as a stable compound.
+
+- **Type**: string
+- **Requirements/Conventions**:
+
+  - **Support**: OPTIONAL support in implementations, i.e., MAY be :val:`null`.
+  - **Query**: MUST be a queryable property with support for all mandatory filter features.
+
+  - If the property is :val:`null` or omitted, no information is provided regarding these aspects of the origin of the structural information.
+    This value is used if the origin of the structural data is not known, or, if it is partially known but to an insufficient degree to distinguish between the categories defined below.
+
+  - If present and not :val:`null`, the property SHOULD take the first of the following values for which the structural information qualifies:
+
+    * :val:`experimental`: the structural information is a faithful representation of the outcome of an experimental technique for structure determination.
+
+    * :val:`derived`: the structural information originates from experimental data, but has undergone additional processing in such a way that the result is still recognizable as the experimental structure it was based on.
+      For example, experimental structures relaxed using *ab initio* calculations are meant to qualify for this category.
+      Structures where one or more elements in a structure have been substituted (while, e.g., keeping the experimental coordinates the same) are not meant to qualify for this category.
+      The category definition involves a degree of subjectivity that has to be decided by the database provider.
+
+    * :val:`predicted`: the structural information is not directly related to the outcome of an experiment on an existing material, but is proposed from theoretical methods to represent a potentially synthesizable structure.
+      For example, theoretically invented structures found to be close to the convex hull of thermodynamical stability at reasonable conditions by relaxation using *ab initio* calculations, AI models with a demonstrated reasonable predictive power, or similar techniques qualify for this category.
+      Structures that are the outcome of theoretical methods that specifically target conditions far outside normal conditions are not meant to qualify for this category.
+      This category definition involves a degree of subjectivity that has to be determined by the database provider.
+      It is OPTIONAL for database providers to predict theoretical structures as potentially synthesizable or not. 
+      If this functionality is not provided, any structures that could fit this category are meant to be assigned the next qualifying category below instead.
+
+    * :val:`hypothetical`: the structural information is not directly related to the outcome of an experiment on an existing material, but is proposed from theoretical methods to represent a local energy minimum on the potential energy surface.
+      This category accommodates potentially highly thermodynamically unstable structures, e.g., predicted to decompose into other competing phases, even with respect to the elemental solids.
+      For example, structures relaxed using *ab initio* calculations but not identified to end up close to the convex hull of thermodynamical stability qualify for this category.
+      AI models which, with a demonstrated reasonable predictive power, can generate structures in local potential energy minimums (e.g., by relaxation via predicted forces, direct generation, etc.) also qualify.
+      This category definition also involves a degree of subjectivity that has to be determined by the database provider.
+
+    * :val:`arbitrary`: the structural information has been created in a way that does not ensure any type of stability, i.e., not even representing a local energy minimum on the potential energy surface.
+      For example, arbitrarily placed atoms that have not been locally optimized, non-equilibrium snapshots, and outcomes of AI models for which the predictive power is deemed insufficient for the earlier categories, all qualify for this category.
+
+    * :val:`other`: the origin of the structural information is known, but is not correctly described by any of the above categories.
+
+  - Database-specific strings using a database provider prefix (e.g., `_exmpl_experimental_at_extreme_pressure`) MAY be used but are strongly discouraged.
+    Clients encountering unrecognized strings SHOULD treat them to mean the same as the field having the value :val:`null`.
+
+- **Examples**:
+
+  - For a structure entry directly encoding structural information obtained from a neutron diffraction experiment: :val:`"experimental"`.
+  - For a structure entry that encodes the structural information from a theoretical relaxation of an "experimental" entry using computational software that implements density functional theory: :val:`"derived"`.
+
 Calculations Entries
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