def update_bond(psi,
b,
wf,
ortho,
normalize=True,
max_truncation_error=None,
max_bond_dim=None):
"""Update the MPS tensors at bond b using the two-site wave-function wf.
If the MPS orthogonality center is at site b or b+1,the canonical form
will be preserved with the new orthogonality center position depending on the value
of ortho.
Args:
psi: The MPS for which the.
b: The bond to update.
wf: The two-site wave-function, it is assumed that wf is a tensornetwork.Node of the form
s_b s_b+1
| |
bond b-1 ---- wf ----- bond b+1
where the edges order of wf is [bond b-1 , s_b, s_b+1, bond b+1]
ortho: 'left' or 'right', on which side of the bond should the orthogonality center
be located after the update.
normalize: Whether to keep the wave-function normalized after update.
max_truncation_error: The maximal allowed truncation error when discarding singular values.
max_bond_dim: An upper bound on the number of kept singular values values.
Returns:
trunc_svals: A list of discarded singular values.
"""
U, S, V, trunc_svals = tn.split_node_full_svd(
wf, [wf[0], wf[1]], [wf[2], wf[3]],
max_truncation_err=max_truncation_error,
max_singular_values=max_bond_dim)
S.set_tensor(S.tensor / tn.norm(S))
if ortho == 'left':
U = U @ S
if psi.center_position == b + 1:
psi.center_position -= 1
elif ortho == 'right':
V = S @ V
if psi.center_position == b:
psi.center_position += 1
else:
raise ValueError("ortho must be 'left' or 'right'")
tn.disconnect(U[-1])
psi.nodes[b] = U
psi.nodes[b + 1] = V
return trunc_svals
def test_disconnect_edge(backend):
a = tn.Node(np.array([1.0] * 5), "a", backend=backend)
b = tn.Node(np.array([1.0] * 5), "b", backend=backend)
e = tn.connect(a[0], b[0])
assert not e.is_dangling()
dangling_edge_1, dangling_edge_2 = tn.disconnect(e)
assert dangling_edge_1.is_dangling()
assert dangling_edge_2.is_dangling()
assert a.get_edge(0) == dangling_edge_1
assert b.get_edge(0) == dangling_edge_2
def test_disconnect_dangling_edge_value_error(backend):
a = tn.Node(np.eye(2), backend=backend)
with pytest.raises(ValueError):
tn.disconnect(a[0])
