Properly convert list of masses to AiiDA and ASE

optimade/adapters/structures/aiida.py

@@ -50,6 +50,7 @@ def get_aiida_structure_data(optimade_structure: OptimadeStructure) -> Structure
         symbols = []
         concentration = []
         masses = []
+        mass = 1.0
         for index, chemical_symbol in enumerate(kind.chemical_symbols):
             # NOTE: The non-chemical element identifier "X" is identical to how AiiDA handles this,
             # so it will be treated the same as any other true chemical identifier.
@@ -63,10 +64,11 @@ def get_aiida_structure_data(optimade_structure: OptimadeStructure) -> Structure
 
                 # AiiDA needs a definition for the mass, and for it to be > 0
                 # mass is OPTIONAL for OPTIMADE structures
-                masses.append(kind.mass[index] if kind.mass else 1)
+                masses.append(kind.mass[index] if kind.mass else mass)
+        mass = sum(masses) if masses else mass
 
         structure.append_kind(
-            Kind(symbols=symbols, weights=concentration, mass=masses, name=kind.name)
+            Kind(symbols=symbols, weights=concentration, mass=mass, name=kind.name)
         )
 
     # Add Sites

optimade/adapters/structures/ase.py

@@ -70,9 +70,7 @@ def get_ase_atoms(optimade_structure: OptimadeStructure) -> Atoms:
         current_species = species[species_name]
 
         # Argument above about chemical symbols also holds here
-        mass = None
-        if current_species.mass:
-            mass = current_species.mass[0]
+        mass = sum(current_species.mass) if current_species.mass else None
 
         atoms.append(Atom(symbol=species_name, position=site, mass=mass))