Improve time evolution interface (#280)

* Improve time evolution interface

* Update docstring

* Move `force_3site` into `SimpleUpdate`

* Define `Base.iterate` for SimpleUpdate

* Update tests and some examples

* Fix test

* Minor fixes [skip ci]

* Fix merge regressions

* Restore accidentally commented tests

* Undo unnecessary renames

* Separate SU alg, state, iterator

* Remove `H` from TimeEvolver

* Add test for `timestep`

* Add docstrings

* Update SU examples

* Update tests

* Minor changes

* Add verbosity for TimeEvolver

* Store Hamiltonian in TimeEvolver

* Move convergence check out of TimeEvolver

* Restore docstrings

* Fix `timestep`

* Fix tests

* Fix more tests

* Finer parameters for SUState

* Make timestep test look better

* Allow complex Trotter time step

* Reorder parameters of SUState

* Docstring updates

* Change `gate_bothsides` to `purified`

* Remove `rand` from finite-T SU tests

* Move `InfiniteState`

* Remove functionality to reset `iter` from `timestep`

* Docstring updates

* Reduce unicode usage

* Fix test

Author: Yue-Zhengyuan

examples/heisenberg_su/main.jl

@@ -70,12 +70,11 @@ fix a truncation error (if that can be reached by remaining below `Dbond`):
 
 dts = [1.0e-2, 1.0e-3, 4.0e-4]
 tols = [1.0e-6, 1.0e-8, 1.0e-8]
-maxiter = 10000
+nstep = 10000
 trunc_peps = truncerror(; atol = 1.0e-10) & truncrank(Dbond)
-
+alg = SimpleUpdate(; trunc = trunc_peps, bipartite = true)
 for (dt, tol) in zip(dts, tols)
-    alg = SimpleUpdate(dt, tol, maxiter, trunc_peps)
-    global peps, wts, = simpleupdate(peps, H, alg, wts; bipartite = true)
+    global peps, wts, = time_evolve(peps, H, dt, nstep, alg, wts; tol, check_interval = 500)
 end
 
 md"""

examples/hubbard_su/main.jl

@@ -66,26 +66,24 @@ maxiter = 20000
 
 for (dt, tol, Dbond) in zip(dts, tols, Ds)
     trunc = truncerror(; atol = 1.0e-10) & truncrank(Dbond)
-    alg = SimpleUpdate(dt, tol, maxiter, trunc)
-    global peps, wts, = simpleupdate(
-        peps, H, alg, wts; bipartite = false, check_interval = 2000
-    )
+    alg = SimpleUpdate(; trunc, bipartite = false)
+    global peps, wts, = time_evolve(peps, H, dt, maxiter, alg, wts; tol, check_interval = 2000)
 end
 
 md"""
 ## Computing the ground-state energy
 
 In order to compute the energy expectation value with evolved PEPS, we need to converge a
-CTMRG environment on it. We first converge an environment with a small enviroment dimension
-and then use that to initialize another run with bigger environment dimension. We'll use
-`trunc=truncrank(χ)` for that such that the dimension is increased during the second CTMRG
-run:
+CTMRG environment on it. We first converge an environment with a small enviroment dimension,
+which is initialized using the simple update bond weights. Next we use it to initialize 
+another run with bigger environment dimension. The dynamic adjustment of environment dimension
+is achieved by using `trunc=truncrank(χ)` with different `χ`s in the CTMRG runs:
 """
 
 χenv₀, χenv = 6, 16
 env_space = Vect[fℤ₂](0 => χenv₀ / 2, 1 => χenv₀ / 2)
 normalize!.(peps.A, Inf)
-env = CTMRGEnv(rand, Float64, peps, env_space)
+env = CTMRGEnv(wts, peps)
 for χ in [χenv₀, χenv]
     global env, = leading_boundary(
         env, peps; alg = :sequential, tol = 1.0e-8, maxiter = 50, trunc = truncrank(χ)

examples/j1j2_su/main.jl

@@ -48,15 +48,16 @@ Therefore, we shall gradually increase $J_2 / J_1$ from 0.1 to 0.5, each time in
 on the previously evolved PEPS:
 """
 
-dt, tol, maxiter = 1.0e-2, 1.0e-8, 30000
+dt, tol, nstep = 1.0e-2, 1.0e-8, 30000
 check_interval = 4000
 trunc_peps = truncerror(; atol = 1.0e-10) & truncrank(Dbond)
-alg = SimpleUpdate(dt, tol, maxiter, trunc_peps)
+alg = SimpleUpdate(; trunc = trunc_peps)
 for J2 in 0.1:0.1:0.5
-    H = real( ## convert Hamiltonian `LocalOperator` to real floats
+    ## convert Hamiltonian `LocalOperator` to real floats
+    H = real(
         j1_j2_model(ComplexF64, symm, InfiniteSquare(Nr, Nc); J1, J2, sublattice = false),
     )
-    global peps, wts, = simpleupdate(peps, H, alg, wts; check_interval)
+    global peps, wts, = time_evolve(peps, H, dt, nstep, alg, wts; tol, check_interval)
 end
 
 md"""
@@ -69,8 +70,7 @@ tols = [1.0e-9, 1.0e-9]
 J2 = 0.5
 H = real(j1_j2_model(ComplexF64, symm, InfiniteSquare(Nr, Nc); J1, J2, sublattice = false))
 for (dt, tol) in zip(dts, tols)
-    alg′ = SimpleUpdate(dt, tol, maxiter, trunc_peps)
-    global peps, wts, = simpleupdate(peps, H, alg′, wts; check_interval)
+    global peps, wts, = time_evolve(peps, H, dt, nstep, alg, wts; tol)
 end
 
 md"""

src/PEPSKit.jl

@@ -74,6 +74,7 @@ include("algorithms/truncation/fullenv_truncation.jl")
 include("algorithms/truncation/bond_truncation.jl")
 
 include("algorithms/time_evolution/evoltools.jl")
+include("algorithms/time_evolution/time_evolve.jl")
 include("algorithms/time_evolution/simpleupdate.jl")
 include("algorithms/time_evolution/simpleupdate3site.jl")
 
@@ -104,7 +105,8 @@ export fixedpoint
 
 export absorb_weight
 export ALSTruncation, FullEnvTruncation
-export su_iter, su3site_iter, simpleupdate, SimpleUpdate
+export SimpleUpdate
+export TimeEvolver, timestep, time_evolve
 
 export InfiniteSquareNetwork
 export InfinitePartitionFunction

src/algorithms/time_evolution/evoltools.jl

@@ -1,4 +1,22 @@
-const InfiniteState = Union{InfinitePEPS, InfinitePEPO}
+"""
+Process the Trotter time step `dt` according to the intended usage.
+"""
+function _get_dt(
+        state::InfiniteState, dt::Number, imaginary_time::Bool
+    )
+    # PEPS update: exp(-H dt)|ψ⟩
+    # PEPO update (purified): exp(-H dt/2)|ρ⟩
+    # PEPO update (not purified): exp(-H dt/2) ρ exp(-H dt/2)
+    dt′ = (state isa InfinitePEPS) ? dt : (dt / 2)
+    if (state isa InfinitePEPO)
+        @assert size(state)[3] == 1
+    end
+    if !imaginary_time
+        @assert (state isa InfinitePEPS) "Real time evolution of InfinitePEPO (Heisenberg picture) is not implemented."
+        dt′ = 1.0im * dt′
+    end
+    return dt′
+end
 
 function MPSKit.infinite_temperature_density_matrix(H::LocalOperator)
     T = scalartype(H)