Fix 3-site SU dt and reduce artificial C4v breaking

src/algorithms/time_evolution/evoltools.jl

@@ -181,27 +181,28 @@
         c      c+1
 ```
 """
-function _get_gatempo_se(gate::LocalOperator, row::Int, col::Int)
-    Nr, Nc = size(gate.lattice)
+function _get_gatempo_se(ham::LocalOperator, dt::Number, row::Int, col::Int)
+    Nr, Nc = size(ham.lattice)
     @assert 1 <= row <= Nr && 1 <= col <= Nc
-    unit = id(space(gate.terms[1].second, 1))
+    unit = id(space(ham.terms[1].second, 1))
     sites = (
         CartesianIndex(row, col),
         CartesianIndex(row, col + 1),
         CartesianIndex(row - 1, col + 1),
     )
-    nb1x = get_gateterm(gate, (sites[1], sites[2]))
-    nb1y = get_gateterm(gate, (sites[2], sites[3]))
-    nb2 = get_gateterm(gate, (sites[1], sites[3]))
+    nb1x = get_gateterm(ham, (sites[1], sites[2]))
+    nb1y = get_gateterm(ham, (sites[2], sites[3]))
+    nb2 = get_gateterm(ham, (sites[1], sites[3]))
     op = (1 / 2) * (nb1x ⊗ unit + unit ⊗ nb1y) + permute(nb2 ⊗ unit, ((1, 3, 2), (4, 6, 5)))
+    op = exp(-dt * op)
     return gate_to_mpo3(op)
 end
 
 """
 Construct the 3-site gate MPOs on the southeast cluster 
 for 3-site simple update on square lattice.
 """
-function _get_gatempos_se(gate::LocalOperator)
-    Nr, Nc = size(gate.lattice)
-    return collect(_get_gatempo_se(gate, r, c) for r in 1:Nr, c in 1:Nc)
+function _get_gatempos_se(ham::LocalOperator, dt::Number)
+    Nr, Nc = size(ham.lattice)
+    return collect(_get_gatempo_se(ham, dt, r, c) for r in 1:Nr, c in 1:Nc)
 end

src/algorithms/time_evolution/simpleupdate3site.jl

@@ -414,13 +414,13 @@
         ),
     )
     peps2 = deepcopy(peps)
-    for i in 1:2
+    for i in 1:4
         for site in CartesianIndices(peps2.vertices)
             r, c = site[1], site[2]
             gs = gatempos[i][r, c]
             _su3site_se!(r, c, gs, peps2, alg)
         end
-        peps2 = (i == 1) ? rotl90(peps2) : rotr90(peps2)
+        peps2 = rotl90(peps2)
     end
     return peps2
 end
@@ -432,9 +432,15 @@
     peps::InfiniteWeightPEPS, ham::LocalOperator, alg::SimpleUpdate; check_interval::Int=500
 )
     time_start = time()
-    gate = get_expham(alg.dt, ham)
-    # convert gates to 3-site MPOs
-    gatempos = [_get_gatempos_se(gate), _get_gatempos_se(rotl90(gate))]
+    # Convert Hamiltonian to 3-site exponentiated gate MPOs.
+    # Since each bond is updated 4 times, 
+    # `dt` for each MPO should be divided by 4
+    gatempos = [
+        _get_gatempos_se(ham, alg.dt / 4),
+        _get_gatempos_se(rotl90(ham), alg.dt / 4),
+        _get_gatempos_se(rot180(ham), alg.dt / 4),
+        _get_gatempos_se(rotr90(ham), alg.dt / 4),
+    ]
     wtdiff = 1.0
     wts0 = deepcopy(peps.weights)
     for count in 1:(alg.maxiter)