Define machine learning / MatPES schemas (#1222)

* Add schemas for ML training and specifically for the MP co-led MatPES project

* Make matcalc optional

Author: esoteric-ephemera

emmet-builders/emmet/builders/materials/ml.py

@@ -3,14 +3,26 @@
 
 from maggma.builders.map_builder import MapBuilder
 from maggma.core import Store
-from matcalc import PESCalculator
+
+try:
+    from matcalc import PESCalculator
+
+    matcalc_installed = True
+except ImportError:
+    matcalc_installed = False
+
 from pymatgen.core import Structure
 
 from emmet.core.ml import MLDoc
 from emmet.core.utils import jsanitize
 
 if TYPE_CHECKING:
-    from ase.calculators.calculator import Calculator
+    try:
+        from ase.calculators.calculator import Calculator
+
+        ase_installed = True
+    except ImportError:
+        ase_installed = False
 
 
 class MLBuilder(MapBuilder):
@@ -40,6 +52,10 @@ def __init__(
                 MLDocs. Will be saved in each document so use sparingly. Defaults to None.
                 Set to {} to disable default provenance model, version, matcalc_version.
         """
+
+        if not matcalc_installed or not ase_installed:
+            raise ImportError("Please `pip install matcalc` to use the MLBuilder.")
+
         self.materials = materials
         self.ml_potential = ml_potential
         self.kwargs = kwargs

emmet-core/emmet/core/math.py

@@ -23,3 +23,19 @@
 
 ListMatrix3D = List[ListVector3D]
 ListMatrix3D.__doc__ = "Real space Matrix as list"  # type: ignore
+
+VOIGT_INDICES = [(0, 0), (1, 1), (2, 2), (1, 2), (0, 2), (0, 1)]
+
+
+def matrix_3x3_to_voigt(matrix: Matrix3D) -> list[float]:
+    """Convert a 3x3 symmetric matrix to its Voigt vector representation.
+
+    Parameters
+    -----------
+    matrix : Matrix3D
+
+    Returns
+    -----------
+    list of float
+    """
+    return [matrix[idx[0]][idx[1]] for idx in VOIGT_INDICES]