Implement AlternativeSpinHamiltonian for Issue #179

check_my_spin.py

@@ -0,0 +1,16 @@
+from moha.hamiltonians import AlternativeSpinHamiltonian
+
+def main():
+    print("Testing AlternativeSpinHamiltonian integration...")
+    H = AlternativeSpinHamiltonian(num_spins=3, coupling_constants=1.0)
+
+    print("\nOne-Body Integrals:")
+    one_body = H.generate_one_body_integral(dense=True)
+    print(one_body)
+
+    print("\nTwo-Body Integrals (Shape):")
+    two_body = H.generate_two_body_integral(dense=True)
+    print(two_body.shape)
+
+if __name__ == "__main__":
+    main()

moha/hamiltonians.py

@@ -22,7 +22,8 @@
     "HamPPP",
     "HamHuck",
     "HamHub",
-    "HamHeisenberg"
+    "HamHeisenberg",
+    "AlternativeSpinHamiltonian"
 ]
 
 
@@ -393,8 +394,8 @@ class HamHeisenberg(HamiltonianAPI):
     Models spin-1/2 particles on a lattice with the Hamiltonian:\r
 \r
     .. math::\r
-        \hat{H}_{XXZ} = \sum_p (\mu_p^Z - J_{pp}^{\mathrm{eq}}) S_p^Z 
-        + \sum_{pq} J_{pq}^{\mathrm{ax}} S_p^Z S_q^Z 
+        \hat{H}_{XXZ} = \sum_p (\mu_p^Z - J_{pp}^{\mathrm{eq}}) S_p^Z
+        + \sum_{pq} J_{pq}^{\mathrm{ax}} S_p^Z S_q^Z
         + \sum_{pq} J_{pq}^{\mathrm{eq}} (S_p^+ S_q^- + S_p^- S_q^+)
 \r
     `HamIsing` and `HamRG` are special cases of this class.\r
@@ -694,3 +695,41 @@ def __init__(self,
             J_ax=J_ax,
             connectivity=connectivity
         )
+
+
+class AlternativeSpinHamiltonian(HamHeisenberg):
+    """
+    Alternative approach to Spin Hamiltonians (Issue #179).
+
+    Acts as an adapter to map custom spin topologies directly to the
+    fermion creation/annihilation operators in HamHeisenberg, avoiding
+    dense Kronecker products.
+    """
+    def __init__(self, num_spins, coupling_constants, connectivity=None):
+        self.num_spins = int(num_spins)
+
+        try:
+            J_val = float(np.array(coupling_constants)[0] if np.array(coupling_constants).size > 0 else 1.0)
+        except (TypeError, IndexError):
+            J_val = float(coupling_constants)
+
+        if connectivity is None or len(connectivity) == 0:
+            self.connectivity_list = [(i, i + 1) for i in range(self.num_spins - 1)]
+        else:
+            self.connectivity_list = connectivity
+
+        J_matrix = np.zeros((self.num_spins, self.num_spins))
+        for (i, j) in self.connectivity_list:
+            J_matrix[i, j] = J_val
+            J_matrix[j, i] = J_val
+
+        mu = np.zeros(self.num_spins)
+
+        super().__init__(
+            mu=mu,
+            J_eq=J_matrix,
+            J_ax=J_matrix
+        )
+
+
+

moha/test/test_alternative_spin.py

@@ -0,0 +1,25 @@
+import pytest
+import numpy as np
+from moha.hamiltonians import AlternativeSpinHamiltonian
+
+def test_alternative_spin_initialization():
+    """Verify that the adapter correctly maps inputs to J matrices."""
+    H = AlternativeSpinHamiltonian(num_spins=3, coupling_constants=1.0)
+
+    assert H.num_spins == 3
+    assert H.J_eq.shape == (3, 3)
+    assert H.J_ax.shape == (3, 3)
+    # Check that adjacent sites are coupled
+    assert H.J_eq[0, 1] == 1.0
+    assert H.J_eq[1, 0] == 1.0
+
+def test_alternative_spin_integral_generation():
+    """Verify the fermion mapping generates the correct tensor shapes."""
+    H = AlternativeSpinHamiltonian(num_spins=3, coupling_constants=1.0)
+
+    one_body = H.generate_one_body_integral(dense=True)
+    two_body = H.generate_two_body_integral(dense=True)
+
+    # 3 sites means 6 spin-orbitals (3 alpha, 3 beta)
+    assert one_body.shape == (6, 6)
+    assert two_body.shape == (6, 6, 6, 6)