Fix orb model selection

Author: ElliottKasoar

janus_core/helpers/mlip_calculators.py

@@ -263,15 +263,20 @@ def choose_calculator(
         from orb_models.forcefield.graph_regressor import GraphRegressor
         import orb_models.forcefield.pretrained as orb_ff
 
-        model = getattr(orb_ff, model_path.sub("-", "_"), None)()
-        if model is None:
-            raise ValueError(
-                f"Please specify `model_path`, as there is no default model for {arch}"
-            )
+        # Default model
+        model_path = model_path if model_path else "orb_v2"
+
         if isinstance(model_path, GraphRegressor):
             model = model_path
+            model_path = "loaded_GraphRegressor"
         else:
-            model = orb_ff.orb_v2()
+            try:
+                model = getattr(orb_ff, model_path.replace("-", "_"))()
+            except AttributeError as e:
+                raise ValueError(
+                    "`model_path` must be a `GraphRegressor`, pre-trained model label "
+                    "(e.g. 'orb-v2'), or `None` (uses default, orb-v2)"
+                ) from e
 
         calculator = ORBCalculator(model=model, device=device, **kwargs)