fix issue #4

Author: DavitKhach

variational_quantum_eigensolver.ipynb

@@ -170,7 +170,6 @@
     }
    ],
    "source": [
-    "H = hamiltonian_operator(a, b, c, d)\n",
     "exact_result = NumPyEigensolver(H).run()\n",
     "reference_energy = min(np.real(exact_result.eigenvalues))\n",
     "print('The exact ground state energy is: {}'.format(reference_energy))"
@@ -606,7 +605,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.10"
+   "version": "3.6.12"
   }
  },
  "nbformat": 4,