# ITensorTDVP has issues with svd when using new GPU backend

**URL:** <https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477>\
**Category:** ITensor Julia Questions\
**Created:** [February 13, 2024, 9:02pm UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477 "2024-02-13T21:02:44Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![kwang0](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/kwang0/32/290_2.png) [@kwang0](https://itensor.discourse.group/u/kwang0)\
**Post date:** [February 13, 2024, 9:02pm UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/1 "2024-02-13T21:02:44Z")

</div>

I am copying this over from a discussion on [this post](https://itensor.discourse.group/t/ann-change-to-keyword-argument-behavior-in-itensors-jl/1258).

I am trying to use the new GPU backend (`CUDA`) with `ITensorTDVP` and am running into an issue with `svd` when I simply run `tdvp` on an MPS loaded onto the GPU. I believe it has something to do with the updates to the keyword arguments for `svd`, specifically for the handling of the `alg` keyword. Do you know of any quick fixes to this issue?

Here is some minimal code that reproduces the issue.

```auto
using ITensors
using ITensorTDVP
using CUDA

N = 10
cutoff = 1e-12

s = siteinds("S=1/2", N)

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

H = cu(MPO(heisenberg(N), s))

ψ0 = cu(randomMPS(s; linkdims=10))

ψ1 = tdvp(H, -0.1im, ψ0; cutoff)

```

This only seems to be a problem for two-site TDVP however. If I set `nsite=1` in the `tdvp()` call then there is no issue.

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<div class="post-metadata">

**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\
**Post date:** [February 13, 2024, 10:52pm UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/2 "2024-02-13T22:52:25Z")

</div>

Thanks for the report, we’ll look into it. Could you also post the full error message you get?

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<div class="post-metadata">

**Author:** ![kwang0](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/kwang0/32/290_2.png) [@kwang0](https://itensor.discourse.group/u/kwang0)\
**Post date:** [February 14, 2024, 12:18am UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/3 "2024-02-14T00:18:23Z")

</div>

Thank you, and yes here is the error message:

```auto
ERROR: LoadError: TypeError: in keyword argument alg, expected String, got a value of type LinearAlgebra.DivideAndConquer
Stacktrace:
  [1] svd(T::NDTensors.DenseTensor{ComplexF64, 2, Tuple{Index{Int64}, Index{Int64}}, NDTensors.Dense{ComplexF64, CuArray{ComplexF64, 1, CUDA.Mem.DeviceBuffer}}}; mindim::Int64, maxdim::Int64, cutoff::Float64, use_absolute_cutoff::Nothing, use_relative_cutoff::Nothing, alg::String, min_blockdim::Nothing)  
    @ NDTensors C:\Users\Kevin\.julia\packages\NDTensors\9WMtv\src\linearalgebra\linearalgebra.jl:88  
  [2] svd(A::ITensor, Linds::Tuple{Index{Int64}, Index{Int64}}; leftdir::Nothing, rightdir::Nothing, lefttags::TagSet, righttags::TagSet, mindim::Int64, maxdim::Int64, cutoff::Float64, alg::String, use_absolute_cutoff::Nothing, use_relative_cutoff::Nothing, min_blockdim::Nothing, utags::TagSet, vtags::TagSet)
    @ ITensors C:\Users\Kevin\.julia\packages\ITensors\7KqSL\src\tensor_operations\matrix_decomposition.jl:162
  [3] factorize_svd(A::ITensor, Linds::Tuple{Index{Int64}, Index{Int64}}; singular_values!::Nothing, ortho::String, alg::String, dir::Nothing, mindim::Int64, maxdim::Int64, cutoff::Float64, tags::TagSet, 
use_absolute_cutoff::Nothing, use_relative_cutoff::Nothing, min_blockdim::Nothing)
    @ ITensors C:\Users\Kevin\.julia\packages\ITensors\7KqSL\src\tensor_operations\matrix_decomposition.jl:614
  [4] factorize(A::ITensor, Linds::Tuple{Index{Int64}, Index{Int64}}; mindim::Int64, maxdim::Int64, cutoff::Float64, ortho::String, tags::TagSet, plev::Nothing, which_decomp::Nothing, eigen_perturbation::Nothing, svd_alg::String, use_absolute_cutoff::Nothing, use_relative_cutoff::Nothing, min_blockdim::Nothing, singular_values!::Nothing, dir::Nothing)
    @ ITensors C:\Users\Kevin\.julia\packages\ITensors\7KqSL\src\tensor_operations\matrix_decomposition.jl:808
  [5] replacebond!(M::MPS, b::Int64, phi::ITensor; normalize::Bool, swapsites::Nothing, ortho::String, which_decomp::Nothing, mindim::Int64, maxdim::Int64, cutoff::Float64, eigen_perturbation::Nothing, svd_alg::String, use_absolute_cutoff::Nothing, use_relative_cutoff::Nothing, min_blockdim::Nothing)     
    @ ITensors C:\Users\Kevin\.julia\packages\ITensors\7KqSL\src\mps\mps.jl:559
  [6] tdvp_site_update!(nsite_val::Val{2}, reverse_step_val::Val{true}, solver::ITensorTDVP.var"#solver#34"{ITensorTDVP.var"#solver#33#35"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, PH::ProjMPO, psi::MPS, b::Int64; current_time::Float64, outputlevel::Int64, time_step::ComplexF64, normalize::Bool, direction::Base.Order.ForwardOrdering, noise::Float64, which_decomp::Nothing, svd_alg::String, cutoff::Float64, maxdim::Int64, mindim::Int64, maxtruncerr::Float64)      
    @ ITensorTDVP C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_step.jl:365
  [7] tdvp_site_update!(solver::ITensorTDVP.var"#solver#34"{ITensorTDVP.var"#solver#33#35"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, PH::ProjMPO, psi::MPS, b::Int64; nsite::Int64, reverse_step::Bool, current_time::Float64, outputlevel::Int64, time_step::ComplexF64, normalize::Bool, direction::Base.Order.ForwardOrdering, noise::Float64, which_decomp::Nothing, svd_alg::String, cutoff::Float64, maxdim::Int64, mindim::Int64, maxtruncerr::Float64)
    @ ITensorTDVP C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_step.jl:157
  [8] tdvp_sweep(direction::Base.Order.ForwardOrdering, solver::Function, PH::ProjMPO, time_step::ComplexF64, psi::MPS; kwargs::Base.Pairs{Symbol, Real, NTuple{7, Symbol}, NamedTuple{(:current_time, :cutoff, :reverse_step, :sweep, :maxdim, :mindim, :noise), Tuple{Float64, Float64, Bool, Int64, Int64, Int64, Float64}}})
    @ ITensorTDVP C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_step.jl:80
  [9] tdvp_step(order::ITensorTDVP.TDVPOrder{2, Base.Order.ForwardOrdering()}, solver::Function, PH::ProjMPO, time_step::ComplexF64, psi::MPS; current_time::Float64, kwargs::Base.Pairs{Symbol, Real, NTuple{6, Symbol}, NamedTuple{(:cutoff, :reverse_step, :sweep, :maxdim, :mindim, :noise), Tuple{Float64, Bool, Int64, Int64, Int64, Float64}}})
    @ ITensorTDVP C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_step.jl:9
 [10] macro expansion
    @ C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_generic.jl:84 [inlined]
 [11] macro expansion
    @ .\timing.jl:382 [inlined]
 [12] tdvp(solver::Function, PH::ProjMPO, t::ComplexF64, psi0::MPS; kwargs::Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}})
    @ ITensorTDVP C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_generic.jl:83
 [13] tdvp(solver::Function, H::MPO, t::ComplexF64, psi0::MPS; kwargs::Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}})
    @ ITensorTDVP C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp_generic.jl:150
 [14] #tdvp#41
    @ C:\Users\Kevin\.julia\packages\ITensorTDVP\V1Kco\src\tdvp.jl:47 [inlined]
 [15] top-level scope
    @ C:\Users\Kevin\Documents\VS Code\QSL\test.jl:25
 [16] include(fname::String)
    @ Base.MainInclude .\client.jl:476
 [17] top-level scope
    @ REPL[62]:1
 [18] top-level scope
    @ C:\Users\Kevin\.julia\packages\CUDA\rXson\src\initialization.jl:208
in expression starting at C:\Users\Kevin\Documents\VS Code\QSL\test.jl:25

```

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<div class="post-metadata">

**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\
**Post date:** [February 14, 2024, 1:09am UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/4 "2024-02-14T01:09:51Z")

</div>

Thanks. Could you also print the versions of the packages you are using?

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**Author:** ![kmp5](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/kmp5/32/251_2.png) [@kmp5](https://itensor.discourse.group/u/kmp5)\
**Post date:** [February 14, 2024, 3:14am UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/5 "2024-02-14T03:14:52Z")

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Hi,

Hi, Thank you for the report. I have sent Matt a message about where the problem comes from and how to fix it in this package. In the meantime, you can utilize CUDA in the code you wrote by modifying the last line to look like this

```auto
ψ1 = tdvp(H, -0.1im, ψ0; cutoff, svd_alg="qr_algorithm")

```

---

<div class="post-metadata">

**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\
**Post date:** [February 15, 2024, 3:32am UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/6 "2024-02-15T03:32:21Z")

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This will be fixed by [Refactor keyword argument processing by mtfishman · Pull Request #62 · ITensor/ITensorTDVP.jl · GitHub](https://github.com/ITensor/ITensorTDVP.jl/pull/62), I will register ITensorTDVP v0.2 tomorrow which includes a fix for this issue.

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**Author:** ![kwang0](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/kwang0/32/290_2.png) [@kwang0](https://itensor.discourse.group/u/kwang0)\
**Post date:** [February 15, 2024, 6:05pm UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/7 "2024-02-15T18:05:14Z")

</div>

Great, thanks everyone!

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<div class="post-metadata">

**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\
**Post date:** [February 16, 2024, 6:55pm UTC](https://itensor.discourse.group/t/itensortdvp-has-issues-with-svd-when-using-new-gpu-backend/1477/8 "2024-02-16T18:55:49Z")

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Should be fixed now in the latest ITensorTDVP version.