Add new Observer that gets updated after each time step (#22)

examples/04_tdvp_observers.jl

@@ -20,7 +20,7 @@ ttotal = 1.0
 s = siteinds("S=1/2", N; conserve_qns=true)
 H = MPO(heisenberg(N), s)
 
-function sweep(; sweep, bond, half_sweep)
+function step(; sweep, bond, half_sweep)
   if bond == 1 && half_sweep == 2
     return sweep
   end
@@ -49,10 +49,10 @@ function return_state(; psi, bond, half_sweep)
 end
 
 obs = Observer(
-  "sweeps" => sweep, "times" => current_time, "psis" => return_state, "Sz" => measure_sz
+  "steps" => step, "times" => current_time, "psis" => return_state, "Sz" => measure_sz
 )
 
-psi = productMPS(s, n -> isodd(n) ? "Up" : "Dn")
+psi = MPS(s, n -> isodd(n) ? "Up" : "Dn")
 psi_f = tdvp(
   H,
   -im * ttotal,
@@ -65,14 +65,15 @@ psi_f = tdvp(
 )
 
 res = results(obs)
-
-sweeps = res["sweeps"]
+steps = res["steps"]
 times = res["times"]
 psis = res["psis"]
 Sz = res["Sz"]
 
-for n in 1:length(sweeps)
-  print("sweep = ", sweeps[n])
+println("\nResults")
+println("=======")
+for n in 1:length(steps)
+  print("step = ", steps[n])
   print(", time = ", round(times[n]; digits=3))
   print(", |⟨ψⁿ|ψⁱ⟩| = ", round(abs(inner(psis[n], psi)); digits=3))
   print(", |⟨ψⁿ|ψᶠ⟩| = ", round(abs(inner(psis[n], psi_f)); digits=3))

examples/05_tdvp_nonuniform_timesteps.jl

@@ -0,0 +1,47 @@
+using ITensors
+using ITensorTDVP
+
+include("05_utils.jl")
+
+function heisenberg(N)
+  os = OpSum()
+  for j in 1:(N - 1)
+    os += 0.5, "S+", j, "S-", j + 1
+    os += 0.5, "S-", j, "S+", j + 1
+    os += "Sz", j, "Sz", j + 1
+  end
+  return os
+end
+
+N = 10
+cutoff = 1e-12
+outputlevel = 1
+nsteps = 10
+time_steps = [n ≤ 2 ? -0.2im : -0.1im for n in 1:nsteps]
+
+obs = Observer("times" => (; current_time) -> current_time, "psis" => (; psi) -> psi)
+
+s = siteinds("S=1/2", N; conserve_qns=true)
+H = MPO(heisenberg(N), s)
+
+psi0 = MPS(s, n -> isodd(n) ? "Up" : "Dn")
+psi = tdvp_nonuniform_timesteps(
+  ProjMPO(H), psi0; time_steps, cutoff, outputlevel, (step_observer!)=obs
+)
+
+res = results(obs)
+times = res["times"]
+psis = res["psis"]
+
+println("\nResults")
+println("=======")
+print("step = ", 0)
+print(", time = ", zero(ComplexF64))
+print(", ⟨Sᶻ⟩ = ", round(expect(psi0, "Sz"; sites=N ÷ 2); digits=3))
+println()
+for n in 1:length(times)
+  print("step = ", n)
+  print(", time = ", round(times[n]; digits=3))
+  print(", ⟨Sᶻ⟩ = ", round(expect(psis[n], "Sz"; sites=N ÷ 2); digits=3))
+  println()
+end

examples/05_utils.jl

@@ -0,0 +1,60 @@
+using ITensors
+using ITensorTDVP
+using Observers
+using Printf
+
+using ITensorTDVP: tdvp_solver, process_sweeps, TDVPOrder
+
+function tdvp_nonuniform_timesteps(
+  solver,
+  PH,
+  psi::MPS;
+  time_steps,
+  reverse_step=true,
+  time_start=0.0,
+  order=2,
+  (step_observer!)=Observer(),
+  kwargs...,
+)
+  nsweeps = length(time_steps)
+  maxdim, mindim, cutoff, noise = process_sweeps(; nsweeps, kwargs...)
+  tdvp_order = TDVPOrder(order, Base.Forward)
+  current_time = time_start
+  for sw in 1:nsweeps
+    sw_time = @elapsed begin
+      psi, PH, info = tdvp(
+        tdvp_order,
+        solver,
+        PH,
+        time_steps[sw],
+        psi;
+        kwargs...,
+        current_time,
+        reverse_step,
+        sweep=sw,
+        maxdim=maxdim[sw],
+        mindim=mindim[sw],
+        cutoff=cutoff[sw],
+        noise=noise[sw],
+      )
+    end
+    current_time += time_steps[sw]
+
+    update!(step_observer!; psi, sweep=sw, outputlevel, current_time)
+
+    if outputlevel ≥ 1
+      print("After sweep ", sw, ":")
+      print(" maxlinkdim=", maxlinkdim(psi))
+      @printf(" maxerr=%.2E", info.maxtruncerr)
+      print(" current_time=", round(current_time; digits=3))
+      print(" time=", round(sw_time; digits=3))
+      println()
+      flush(stdout)
+    end
+  end
+  return psi
+end
+
+function tdvp_nonuniform_timesteps(H, psi::MPS; kwargs...)
+  return tdvp_nonuniform_timesteps(tdvp_solver(; kwargs...), H, psi; kwargs...)
+end

src/tdvp_generic.jl

@@ -55,7 +55,8 @@ function tdvp(solver, PH, t::Number, psi0::MPS; kwargs...)
   write_when_maxdim_exceeds::Union{Int,Nothing} = get(
     kwargs, :write_when_maxdim_exceeds, nothing
   )
-  observer = get(kwargs, :observer, NoObserver())
+  observer = get(kwargs, :observer!, NoObserver())
+  step_observer = get(kwargs, :step_observer!, NoObserver())
   outputlevel::Int = get(kwargs, :outputlevel, 0)
 
   psi = copy(psi0)
@@ -97,6 +98,8 @@ function tdvp(solver, PH, t::Number, psi0::MPS; kwargs...)
 
     current_time += time_step
 
+    update!(step_observer; psi, sweep=sw, outputlevel, current_time)
+
     if outputlevel >= 1
       print("After sweep ", sw, ":")
       print(" maxlinkdim=", maxlinkdim(psi))

src/tdvp_step.jl

@@ -153,7 +153,7 @@ function tdvp(direction::Base.Ordering, solver, PH, time_step::Number, psi::MPS;
       @printf(" cutoff=%.1E", cutoff)
       @printf(" maxdim=%.1E", maxdim)
       print(" mindim=", mindim)
-      print(" current_time=", current_time)
+      print(" current_time=", round(current_time; digits=3))
       println()
       if spec != nothing
         @printf(

test/test_tdvp.jl

@@ -376,23 +376,34 @@ end
 
   obs = Observer("Sz" => measure_sz, "En" => measure_en)
 
-  psi2 = productMPS(s, n -> isodd(n) ? "Up" : "Dn")
-  tdvp(H, -im * ttotal, psi2; time_step=-im * tau, cutoff, normalize=false, (observer!)=obs)
+  step_measure_sz(; psi) = expect(psi, "Sz"; sites=c)
+
+  step_measure_en(; psi) = real(inner(psi', H, psi))
+
+  step_obs = Observer("Sz" => step_measure_sz, "En" => step_measure_en)
+
+  psi2 = MPS(s, n -> isodd(n) ? "Up" : "Dn")
+  tdvp(
+    H,
+    -im * ttotal,
+    psi2;
+    time_step=-im * tau,
+    cutoff,
+    normalize=false,
+    (observer!)=obs,
+    (step_observer!)=step_obs,
+  )
 
-  # Using filter here just due to the current
-  # behavior of Observers that nothing gets appended:
   Sz2 = results(obs)["Sz"]
   En2 = results(obs)["En"]
 
-  #display(En1)
-  #display(En2)
-  #display(Sz1)
-  #display(Sz2)
-  #@show norm(Sz1 - Sz2)
-  #@show norm(En1 - En2)
+  Sz2_step = results(step_obs)["Sz"]
+  En2_step = results(step_obs)["En"]
 
   @test Sz1 ≈ Sz2
   @test En1 ≈ En2
+  @test Sz1 ≈ Sz2_step
+  @test En1 ≈ En2_step
 end
 
 nothing