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Fixing time_start argument in tdvp#172

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osvathbotond wants to merge 2 commits intoITensor:mainfrom
osvathbotond:feature/tdvp_time_start_fix
Open

Fixing time_start argument in tdvp#172
osvathbotond wants to merge 2 commits intoITensor:mainfrom
osvathbotond:feature/tdvp_time_start_fix

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@osvathbotond osvathbotond commented Feb 8, 2026

Description

In the current tdvp implementation, the "time_start" argument is not working as I'd expect it to. If we set t=10.0, time_start=5.0 and dt=1.0, it'll calculate that it needs 10 steps in total, so it'll go until 15.0 instead of the desired 10.0.

Minimal demonstration of previous behavior

using ITensors, ITensorMPS

using Compat: @compat
using ITensors: ITensor, array, inds, itensor
using ITensorMPS: TimeDependentSum, to_vec
using KrylovKit: exponentiate
using OrdinaryDiffEqTsit5: ODEProblem, Tsit5, solve

function krylov_updater(operator, init; internal_kwargs, kwargs...)
  @compat (; current_time, time_step) = (; current_time=zero(Bool), internal_kwargs...)
  state, info = exponentiate(operator(current_time), time_step, init; kwargs...)
  return state, (; info)
end

function get_sites(N)
    sites = siteinds("S=1/2", N)
    return sites
end

function get_H(Jz, Jxy, sites)
    N = length(sites)
    os = OpSum()
    for j in 1:(N - 1)
        os += Jz, "Sz", j, "Sz", j + 1
        os += 0.5*Jxy, "S+", j, "S-", j + 1
        os += 0.5*Jxy, "S-", j, "S+", j + 1
    end
    H = MPO(os, sites)
    return H
end

function get_psi(H, sites)
    psi0 = random_mps(sites)

    nsweeps = 5
    maxdim = [10, 20, 100, 100, 200]
    cutoff = 1.0e-10

    energy, psi = dmrg(H, psi0; nsweeps, maxdim, cutoff)
    return psi
end

function generate_fs(T)
    function f1(t)
        normalized_time = t * (π / (2 * T))
        f = sin(normalized_time)^2
        return 1-f
    end

    function f2(t)
        normalized_time = t * (π / (2 * T))
        f = sin(normalized_time)^2
        return f
    end

    return f1, f2
end



function main()
    Jz = 1.0
    Jxy = 1.0
    N = 100
    dt = 1.0
    T = 10.0


    sites = get_sites(N)
    H1 = get_H(Jz, 0, sites)
    H2 = get_H(0, Jxy, sites)

    psi_1 = get_psi(H1, sites)

    f1, f2 = generate_fs(T)

    psi_Thalf = tdvp(-im * TimeDependentSum([f1, f2], [H1, H2]), T/2, complex.(psi_1); updater=krylov_updater, nsite=2, time_step=dt, cutoff=1e-5, maxdim=200, outputlevel=1);
    psi_T = tdvp(-im * TimeDependentSum([f1, f2], [H1, H2]), T, complex.(psi_Thalf); updater=krylov_updater, nsite=2, time_step=dt, cutoff=1e-5, maxdim=200, time_start=T/2, outputlevel=1);
end

main()

Output:

After sweep 1 energy=-24.749999996767972  maxlinkdim=2 maxerr=9.24E-11 time=16.436
After sweep 2 energy=-24.75  maxlinkdim=1 maxerr=7.86E-49 time=0.068
After sweep 3 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.057
After sweep 4 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.054
After sweep 5 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.054
After sweep 1: maxlinkdim=1 maxerr=2.36E-06 current_time=1.0 time=25.88
After sweep 2: maxlinkdim=2 maxerr=4.88E-06 current_time=2.0 time=0.278
After sweep 3: maxlinkdim=2 maxerr=1.12E-07 current_time=3.0 time=0.409
After sweep 4: maxlinkdim=2 maxerr=4.69E-06 current_time=4.0 time=0.503
After sweep 5: maxlinkdim=3 maxerr=6.63E-06 current_time=5.0 time=1.065
After sweep 1: maxlinkdim=4 maxerr=1.74E-06 current_time=6.0 time=2.651
After sweep 2: maxlinkdim=5 maxerr=8.41E-06 current_time=7.0 time=3.405
After sweep 3: maxlinkdim=6 maxerr=9.99E-06 current_time=8.0 time=3.241
After sweep 4: maxlinkdim=8 maxerr=9.19E-06 current_time=9.0 time=3.686
After sweep 5: maxlinkdim=11 maxerr=8.79E-06 current_time=10.0 time=4.237
After sweep 6: maxlinkdim=15 maxerr=9.83E-06 current_time=11.0 time=5.449
After sweep 7: maxlinkdim=22 maxerr=9.96E-06 current_time=12.0 time=8.616
After sweep 8: maxlinkdim=33 maxerr=9.66E-06 current_time=13.0 time=16.147
After sweep 9: maxlinkdim=47 maxerr=9.83E-06 current_time=14.0 time=27.608
After sweep 10: maxlinkdim=70 maxerr=9.90E-06 current_time=15.0 time=55.292

Minimal demonstration of new behavior

using ITensors, ITensorMPS

using Compat: @compat
using ITensors: ITensor, array, inds, itensor
using ITensorMPS: TimeDependentSum, to_vec
using KrylovKit: exponentiate
using OrdinaryDiffEqTsit5: ODEProblem, Tsit5, solve

function krylov_updater(operator, init; internal_kwargs, kwargs...)
  @compat (; current_time, time_step) = (; current_time=zero(Bool), internal_kwargs...)
  state, info = exponentiate(operator(current_time), time_step, init; kwargs...)
  return state, (; info)
end

function get_sites(N)
    sites = siteinds("S=1/2", N)
    return sites
end

function get_H(Jz, Jxy, sites)
    N = length(sites)
    os = OpSum()
    for j in 1:(N - 1)
        os += Jz, "Sz", j, "Sz", j + 1
        os += 0.5*Jxy, "S+", j, "S-", j + 1
        os += 0.5*Jxy, "S-", j, "S+", j + 1
    end
    H = MPO(os, sites)
    return H
end

function get_psi(H, sites)
    psi0 = random_mps(sites)

    nsweeps = 5
    maxdim = [10, 20, 100, 100, 200]
    cutoff = 1.0e-10

    energy, psi = dmrg(H, psi0; nsweeps, maxdim, cutoff)
    return psi
end

function generate_fs(T)
    function f1(t)
        normalized_time = t * (π / (2 * T))
        f = sin(normalized_time)^2
        return 1-f
    end

    function f2(t)
        normalized_time = t * (π / (2 * T))
        f = sin(normalized_time)^2
        return f
    end

    return f1, f2
end



function main()
    Jz = 1.0
    Jxy = 1.0
    N = 100
    dt = 1.0
    T = 10.0


    sites = get_sites(N)
    H1 = get_H(Jz, 0, sites)
    H2 = get_H(0, Jxy, sites)

    psi_1 = get_psi(H1, sites)

    f1, f2 = generate_fs(T)

    psi_Thalf = tdvp(-im * TimeDependentSum([f1, f2], [H1, H2]), T/2, complex.(psi_1); updater=krylov_updater, nsite=2, time_step=dt, cutoff=1e-5, maxdim=200, outputlevel=1);
    psi_T = tdvp(-im * TimeDependentSum([f1, f2], [H1, H2]), T, complex.(psi_Thalf); updater=krylov_updater, nsite=2, time_step=dt, cutoff=1e-5, maxdim=200, time_start=T/2, outputlevel=1);
end

main()

Output:

After sweep 1 energy=-24.75  maxlinkdim=1 maxerr=8.01E-11 time=15.110
After sweep 2 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.057
After sweep 3 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.055
After sweep 4 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.058
After sweep 5 energy=-24.75  maxlinkdim=1 maxerr=0.00E+00 time=0.054
After sweep 1: maxlinkdim=1 maxerr=2.36E-06 current_time=1.0 time=25.115
After sweep 2: maxlinkdim=2 maxerr=4.88E-06 current_time=2.0 time=0.283
After sweep 3: maxlinkdim=2 maxerr=1.12E-07 current_time=3.0 time=0.393
After sweep 4: maxlinkdim=2 maxerr=4.69E-06 current_time=4.0 time=0.534
After sweep 5: maxlinkdim=3 maxerr=6.63E-06 current_time=5.0 time=1.098
After sweep 1: maxlinkdim=4 maxerr=1.74E-06 current_time=6.0 time=2.639
After sweep 2: maxlinkdim=5 maxerr=8.41E-06 current_time=7.0 time=3.242
After sweep 3: maxlinkdim=6 maxerr=9.99E-06 current_time=8.0 time=3.255
After sweep 4: maxlinkdim=8 maxerr=9.19E-06 current_time=9.0 time=3.606
After sweep 5: maxlinkdim=11 maxerr=8.79E-06 current_time=10.0 time=4.123

How Has This Been Tested?

I didn't create any new tests.

Checklist:

  • My code follows the style guidelines of this project. Please run using JuliaFormatter; format(".") in the base directory of the repository (~/.julia/dev/ITensorMPS) to format your code according to our style guidelines.
  • I have performed a self-review of my own code.
  • I have commented my code, particularly in hard-to-understand areas.
  • I have added tests that verify the behavior of the changes I made.
  • I have made corresponding changes to the documentation.
  • My changes generate no new warnings.
  • Any dependent changes have been merged and published in downstream modules.

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github-actions bot commented Feb 8, 2026 •
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Your PR no longer requires formatting changes. Thank you for your contribution!

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codecov bot commented Feb 8, 2026 •
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (d13c9e1) to head (e5eb661).

Additional details and impacted files 
@@           Coverage Diff           @@
##             main     #172   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files          54       54           
  Lines        3604     3604           
=======================================
  Hits         3255     3255           
  Misses        349      349
Flag Coverage Δ
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@osvathbotond