Add optimization_type property for structures

optimade.rst

@@ -2846,6 +2846,48 @@ 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 structure is believed to exist in nature or can be synthesized as a compound stable at non-extreme conditions.
+
+- **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.
+  - SHOULD take one of the following values:
+
+    * :val:`experimental`: the structural information is a faithful representation of the outcome of an experimental technique for structure determination.
+
+    * :val:`processed`: 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.
+      Substituting one or more elements in a structure (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 experimental technique on an existing material, but has undergone theoretical processing to suggest it as a candidate for a potentially synthesizable structure.
+      This category includes theoretically invented structures that have been relaxed using *ab initio* calculations and found to be close to the convex hull of stability, or structures generated from AI models with a demonstrated reasonable predictive power.
+      This category definition also involves a degree of subjectivity that has to be determined by the database provider.
+
+    * :val:`hypothetical`: the structural information is known to have been created in a way that provides no guarantees of producing synthesizable structures or structures found in nature.
+      This category is suitable for randomly placed atoms (e.g., meant to provide a starting point for further processing) or outcomes of AI models with predictive power deemed insufficient for the :val:`predicted` category.
+
+    * :val:`other`: the origin of the structural information is not correctly described by any of the other categories.
+
+    * :val:`unknown`: no information is available regarding these aspects of the origin of the structural information.
+
+    The experiments and predictions referred to in the above definitions of the categories refer to existence at non-extreme conditions (i.e., existence around NTP or at lower temperatures) and in a regular atmosphere.
+    Providers who want to communicate structural information about compounds that exist only at unusual or extreme conditions should categorize them as :val:`other` and, if desired, use another facility (e.g., a provider-specific property) to communicate more specific information.
+
+  - If the property is omitted, set to an empty string, or `null` it means the same thing as :val:`unknown`.
+
+  - 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 :val:`unknown`.
+
+- **Examples**:
+
+  - For a structure entry directly encoding structural information obtained from a neutron diffraction experiment: :val:`"experimental"`.
+
 Calculations Entries
 --------------------