Hey @mileva2319! Welcome to the forum !

but say I want to use VQE for a molecule with more complex atoms, CuO for example. When I try to input the molecular geometry, I end up with an error.

I can only guess as to what the error is, but maybe it’s something like this:

```
import pennylane as qml
from pennylane import numpy as np
symbols = ["Cu", "O"]
coords = np.array([[0.0, 3.0, 0.0], [0.0, 0.0, 0.0]])
hamiltonian, qubits = qml.qchem.molecular_hamiltonian(symbols, coords, mult=2)
'''
ValueError: Atoms in {'Cu'} are not supported.
'''
```

As this error suggests, Cu isn’t a supported atom in our differentiable Hartree-Fock solver. However, you can use a different backend — the OpenFermion-PySCF backend — that supports atoms like Cu. You simply need to add a special keyword argument to specify as much:

```
hamiltonian, qubits = qml.qchem.molecular_hamiltonian(symbols, coords, mult=2, method='pyscf')
print(hamiltonian)
'''
<Hamiltonian: terms=172654, wires=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45]>
'''
```

You’ll need to `pip install openfermionpyscf`

to get this to work . Just note that the resulting Hamiltonian is non-differentiable — you can still perform VQE with it, but it will rely on numerical approaches for computing gradients, which can lead to inaccuracies and instability compared to the more robust differentiable Hartree-Fock solver.

Here’s an example with a smaller molecule:

```
symbols = ["H", "O"]
coords = np.array([[0.0, 1.0, 0.0], [0.0, 0.0, 0.0]])
hamiltonian, qubits = qml.qchem.molecular_hamiltonian(symbols, coords, charge=-1, method='pyscf')
dev = qml.device("default.qubit", wires=12)
@qml.qnode(dev)
def circuit(x):
for i in range(12):
qml.RX(x[i], wires=i)
return qml.expval(hamiltonian)
x = np.random.uniform(0, 2*np.pi, size=(12,))
print(circuit(x))
print(qml.grad(circuit)(x))
'''
-54.046642484365606
[10.84198255 10.38715297 2.15646718 1.49026374 1.7440145 -1.07904077
0.58801415 -1.1730143 1.62609713 -1.67494756 0.95318472 0.16550297]
'''
```

Hope this helps!