diff --git a/mess/hamiltonian.py b/mess/hamiltonian.py
index ae644b5..7be73f5 100644
--- a/mess/hamiltonian.py
+++ b/mess/hamiltonian.py
@@ -12,7 +12,7 @@
 from mess.integrals import eri_basis, kinetic_basis, nuclear_basis, overlap_basis
 from mess.interop import to_pyscf
 from mess.mesh import Mesh, density, density_and_grad, xcmesh_from_pyscf
-from mess.orthnorm import canonical
+from mess.orthnorm import symmetric
 from mess.structure import nuclear_energy
 from mess.types import FloatNxN, OrthNormTransform
 from mess.xcfunctional import (
@@ -25,6 +25,40 @@
 )
 
 xcstr = Literal["lda", "pbe", "pbe0", "b3lyp", "hfx"]
+IntegralBackend = Literal["mess", "pyscf_cart", "pyscf_sph"]
+
+
+class OneElectron(eqx.Module):
+    overlap: FloatNxN
+    kinetic: FloatNxN
+    nuclear: FloatNxN
+
+    def __init__(self, basis: Basis, backend: IntegralBackend = "mess"):
+        """_summary_
+
+        Args:
+            basis (Basis): _description_
+            backend (IntegralBackend, optional): _description_. Defaults to "mess".
+
+        Raises:
+            ValueError: _description_
+            ValueError: _description_
+
+        Returns:
+            _type_: _description_
+        """
+        if backend == "mess":
+            self.overlap = overlap_basis(basis)
+            self.kinetic = kinetic_basis(basis)
+            self.nuclear = nuclear_basis(basis).sum(axis=0)
+        elif backend.startswith("pyscf_"):
+            mol = to_pyscf(basis.structure, basis.basis_name)
+            kind = backend.split("_")[1]
+            S = jnp.array(mol.intor(f"int1e_ovlp_{kind}"))
+            N = 1 / jnp.sqrt(jnp.diagonal(S))
+            self.overlap = N[:, jnp.newaxis] * N[jnp.newaxis, :] * S
+            self.kinetic = jnp.array(mol.intor(f"int1e_kin_{kind}"))
+            self.nuclear = jnp.array(mol.intor(f"int1e_nuc_{kind}"))
 
 
 class TwoElectron(eqx.Module):
@@ -40,9 +74,10 @@ def __init__(self, basis: Basis, backend: str = "mess"):
         super().__init__()
         if backend == "mess":
             self.eri = eri_basis(basis)
-        elif backend == "pyscf":
+        elif backend.startswith("pyscf_"):
             mol = to_pyscf(basis.structure, basis.basis_name)
-            self.eri = jnp.array(mol.intor("int2e_cart", aosym="s1"))
+            kind = backend.split("_")[1]
+            self.eri = jnp.array(mol.intor(f"int2e_{kind}", aosym="s1"))
 
     def coloumb(self, P: FloatNxN) -> FloatNxN:
         """Build the Coloumb matrix (classical electrostatic) from the density matrix.
@@ -182,15 +217,17 @@ class Hamiltonian(eqx.Module):
     def __init__(
         self,
         basis: Basis,
-        ont: OrthNormTransform = canonical,
+        ont: OrthNormTransform = symmetric,
         xc_method: xcstr = "lda",
+        backend: IntegralBackend = "pyscf_cart",
     ):
         super().__init__()
         self.basis = basis
-        S = overlap_basis(basis)
+        one_elec = OneElectron(basis, backend=backend)
+        S = one_elec.overlap
         self.X = ont(S)
-        self.H_core = kinetic_basis(basis) + nuclear_basis(basis).sum(axis=0)
-        self.two_electron = TwoElectron(basis, backend="pyscf")
+        self.H_core = one_elec.kinetic + one_elec.nuclear
+        self.two_electron = TwoElectron(basis, backend=backend)
         self.xcfunc = build_xcfunc(xc_method, basis, self.two_electron)
 
     def __call__(self, P: FloatNxN) -> ScalarLike: