def test_expectation_value_multisite():
s = spin_half
psi = mps.MPS.from_singlets(s, 6, [(0, 1), (2, 3), (4, 5)], lonely=[], bc='finite')
SpSm = npc.outer(s.Sp.replace_labels(['p', 'p*'], ['p0', 'p0*']),
s.Sm.replace_labels(['p', 'p*'], ['p1', 'p1*']))
psi1 = psi.copy()
ev = psi.expectation_value(SpSm)
npt.assert_almost_equal(ev, [-0.5, 0., -0.5, 0., -0.5])
env1 = mps.MPSEnvironment(psi1, psi)
ev = env1.expectation_value(SpSm)
npt.assert_almost_equal(ev, [-0.5, 0., -0.5, 0., -0.5])
psi1.apply_local_op(2, SpSm) # multi-site operator
ev = psi1.expectation_value(SpSm) # normalized!
npt.assert_almost_equal(ev, [-0.5, 0., 0.0, 0., -0.5])
env1 = mps.MPSEnvironment(psi1, psi)
ev = env1.expectation_value(SpSm) / psi1.overlap(psi) # normalize
npt.assert_almost_equal(ev, [-0.5, 0., -1., 0., -0.5])
def test_MPSEnvironment():
xxz_pars = dict(L=4, Jxx=1., Jz=1.1, hz=0.1, bc_MPS='finite')
L = xxz_pars['L']
M = XXZChain(xxz_pars)
state = ([0, 1] * L)[:L] # Neel state
psi = mps.MPS.from_product_state(M.lat.mps_sites(), state, bc='finite')
env = mps.MPSEnvironment(psi, psi)
env.get_LP(3, True)
env.get_RP(0, True)
env.test_sanity()
for i in range(4):
ov = env.full_contraction(i) # should be one
print("total contraction on site", i, ": ov = 1. - ", ov - 1.)
assert (abs(abs(ov) - 1.) < 1.e-14)
env.expectation_value('Sz')
