Add tests for H expectation from sampling

tests/test_expectation_samples.py

@@ -0,0 +1,69 @@
+"""
+Test expectation functionality
+"""
+
+# import numpy as np
+import pytest
+from qibo import Circuit, gates
+from qibo.hamiltonians import SymbolicHamiltonian
+from qibo.symbols import X, Z
+
+from qibochem.driver.molecule import Molecule
+from qibochem.measurement.expectation import expectation
+
+
+def test_expectation_z0():
+    """Test from_samples functionality of expectation function"""
+    hamiltonian = SymbolicHamiltonian(Z(0))
+    circuit = Circuit(2)
+    circuit.add(gates.X(0))
+    result = expectation(circuit, hamiltonian, from_samples=True)
+    assert pytest.approx(result) == -1.0
+
+
+def test_expectation_z0z1():
+    """Tests expectation_from_samples for diagonal Hamiltonians (only Z's)"""
+    hamiltonian = SymbolicHamiltonian(Z(0) * Z(1))
+    circuit = Circuit(2)
+    circuit.add(gates.X(0))
+    result = expectation(circuit, hamiltonian, from_samples=True)
+    assert pytest.approx(result) == -1.0
+
+
+def test_expectation_x0():
+    """Tests expectation_from_samples for Hamiltonians with X"""
+    hamiltonian = SymbolicHamiltonian(X(0))
+    circuit = Circuit(2)
+    circuit.add(gates.H(0))
+    result = expectation(circuit, hamiltonian, from_samples=True)
+    assert pytest.approx(result) == 1.0
+
+
+def test_expectation_x0_2():
+    """Test 2 of expectation_from_samples for Hamiltonians with X"""
+    hamiltonian = SymbolicHamiltonian(X(0))
+    circuit = Circuit(2)
+    result = expectation(circuit, hamiltonian, from_samples=True, n_shots=10000)
+    assert pytest.approx(result, abs=0.05) == 0.00
+
+
+def test_h2_hf_energy():
+    """Test HF energy of H2 molecule"""
+    # Hardcoded benchmark results
+    h2_ref_energy = -1.117349035
+
+    h2 = Molecule([("H", (0.0, 0.0, 0.0)), ("H", (0.0, 0.0, 0.7))])
+    try:
+        h2.run_pyscf()
+    except ModuleNotFoundError:
+        h2.run_psi4()
+
+    # JW-HF circuit
+    circuit = Circuit(4)
+    circuit.add(gates.X(_i) for _i in range(2))
+
+    # Molecular Hamiltonian and the HF expectation value
+    hamiltonian = h2.hamiltonian()
+    hf_energy = expectation(circuit, hamiltonian, from_samples=True, n_shots=10000)
+
+    assert h2_ref_energy == pytest.approx(hf_energy, abs=0.005)