allow optional two electron integral in xcfunctional factory

mess/hamiltonian.py

@@ -1,5 +1,6 @@
 # Copyright (c) 2024 Graphcore Ltd. All rights reserved.
-from typing import Tuple, Literal, get_args
+from typing import Literal, Optional, Tuple, get_args
+
 import equinox as eqx
 import jax
 import jax.numpy as jnp
@@ -15,15 +16,14 @@
 from mess.structure import nuclear_energy
 from mess.types import FloatNxN, OrthNormTransform
 from mess.xcfunctional import (
-    lda_correlation_vwn,
-    lda_exchange,
+    gga_correlation_lyp,
     gga_correlation_pbe,
-    gga_exchange_pbe,
     gga_exchange_b88,
-    gga_correlation_lyp,
+    gga_exchange_pbe,
+    lda_correlation_vwn,
+    lda_exchange,
 )
 
-
 xcstr = Literal["lda", "pbe", "pbe0", "b3lyp", "hfx"]
 
 
@@ -146,8 +146,13 @@ def __call__(self, P: FloatNxN) -> ScalarLike:
 
 
 def build_xcfunc(
-    xc_method: xcstr, basis: Basis, two_electron: TwoElectron
+    xc_method: xcstr, basis: Basis, two_electron: Optional[TwoElectron] = None
 ) -> eqx.Module:
+    if two_electron is None and xc_method in ("pbe0", "b3lyp"):
+        raise ValueError(
+            f"Hybrid functional {xc_method} requires providing TwoElectron integrals"
+        )
+
     match xc_method:
         case "lda":
             return LDA(basis)