Fix zero gate returned by get_gateterm (#300)

Author: Yue-Zhengyuan

src/algorithms/time_evolution/evoltools.jl

@@ -85,10 +85,13 @@ function get_gateterm(gate::LocalOperator, bond::NTuple{2, CartesianIndex{2}})
         if length(bonds) == 1
             return permute(gate.terms[bonds[1]].second, ((2, 1), (4, 3)))
         elseif length(bonds) == 0
-            # if term not found, return the zero operator
-            dtype = scalartype(gate.terms[1].second)
-            V = space(gate.terms[1].second, 1)
-            return zeros(dtype, V ⊗ V ← V ⊗ V)
+            # if term not found, return the zero operator on this bond
+            dtype = scalartype(gate)
+            r1, c1 = (mod1(bond[1][i], n) for (i, n) in zip(1:2, size(gate)))
+            r2, c2 = (mod1(bond[2][i], n) for (i, n) in zip(1:2, size(gate)))
+            V1 = physicalspace(gate)[r1, c1]
+            V2 = physicalspace(gate)[r2, c2]
+            return zeros(dtype, V1 ⊗ V2 ← V1 ⊗ V2)
         else
             error("There are multiple terms in `gate` corresponding to the bond $(bond).")
         end