# The ProjMPO-ITensor product error message inside tdvp **URL:** \ **Category:** ITensor Julia Questions\ **Tags:** mpo, tdvp\ **Created:** [May 4, 2025, 4:04am UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368 "2025-05-04T04:04:57Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Zhen\_H](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/zhen_h/32/764_2.png) [@Zhen\_H](https://itensor.discourse.group/u/Zhen_H)\ **Post date:** [May 4, 2025, 4:04am UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/1 "2025-05-04T04:04:57Z")
Hi, I am using the `tdvp` function and it gives me the following error: ```auto ProjMPO-ITensor product P*v is not equal to the order of the ITensor v. ``` I’ve attached the full error message in the end. Note that I have not tweeked anything inside `tdvp`. In other words, I am using `tdvp` straightly. Here is my julia code. I have a `psi` loaded from a h5 file. Then I do the following: ```julia n = 9 sites = siteinds(psi) cdag = op("Cdag", sites, n) psi[n] = psi[n]*cdag psi = tdvp(H, t, psi; time_step=dt) ``` I am a bit stuck on debugging this, and I am wondering if there are any suggestions on what could possibly lead to this error. Maybe it is worth mentioning that if I commented out `psi[n] = psi[n]*cdag`, then the whole thing runs without error. The full error message is as follows: ```auto ERROR: LoadError: The order of the ProjMPO-ITensor product P*v is not equal to the order of the ITensor v, this is probably due to an index mismatch. Common reasons for this error: (1) You are trying to multiply the ProjMPO with the 2-site wave-function at the wrong position. (2) `orthogonalize!` was called, changing the MPS without updating the ProjMPO. P*v inds: ((dim=2|id=845|"Fermion,Site,n=9"), (dim=2|id=845|"Fermion,Site,n=9")'', (dim=4|id=535|"sum")', (dim=2|id=978|"Fermion,Site,n=2")', (dim=2|id=288|"Fermion,Site,n=1")') v inds: ((dim=2|id=288|"Fermion,Site,n=1"), (dim=2|id=978|"Fermion,Site,n=2"), (dim=4|id=535|"sum")) Stacktrace: [1] error(s::String) @ Base ./error.jl:35 [2] product(P::ProjMPO, v::ITensor) @ ITensorMPS ~/.julia/packages/ITensorMPS/hghpU/src/abstractprojmpo/abstractprojmpo.jl:73 [3] AbstractProjMPO @ ~/.julia/packages/ITensorMPS/hghpU/src/abstractprojmpo/abstractprojmpo.jl:87 [inlined] [4] apply @ ~/.julia/packages/KrylovKit/jC5gU/src/apply.jl:2 [inlined] [5] expintegrator(A::ProjMPO, t::Float64, u::Tuple{ITensor, ITensor}, alg::KrylovKit.Arnoldi{KrylovKit.ModifiedGramSchmidt2, Float64}) @ KrylovKit ~/.julia/packages/KrylovKit/jC5gU/src/matrixfun/expintegrator.jl:109 [6] expintegrator @ ~/.julia/packages/KrylovKit/jC5gU/src/matrixfun/expintegrator.jl:102 [inlined] [7] expintegrator(::ProjMPO, ::Float64, ::ITensor; kwargs::@Kwargs{}) @ KrylovKit ~/.julia/packages/KrylovKit/jC5gU/src/matrixfun/expintegrator.jl:98 [8] expintegrator @ ~/.julia/packages/KrylovKit/jC5gU/src/matrixfun/expintegrator.jl:94 [inlined] [9] exponentiate @ ~/.julia/packages/KrylovKit/jC5gU/src/matrixfun/exponentiate.jl:83 [inlined] [10] #exponentiate_updater#765 @ ~/.julia/packages/ITensorMPS/hghpU/src/solvers/tdvp.jl:5 [inlined] [11] exponentiate_updater @ ~/.julia/packages/ITensorMPS/hghpU/src/solvers/tdvp.jl:4 [inlined] [12] region_update!(nsite_val::Val{2}, reverse_step_val::Val{true}, reduced_operator::ProjMPO, state::MPS, b::Int64; updater::typeof(ITensorMPS.exponentiate_updater), updater_kwargs::@NamedTuple{}, current_time::Float64, time_step::Float64, outputlevel::Int64, normalize::Bool, direction::Base.Order.ForwardOrdering, noise::Bool, which_decomp::Nothing, svd_alg::Nothing, cutoff::Float64, maxdim::Int64, mindim::Int64, maxtruncerr::Float64) @ ITensorMPS ~/.julia/packages/ITensorMPS/hghpU/src/solvers/sweep_update.jl:408 [13] region_update!(reduced_operator::ProjMPO, state::MPS, b::Int64; updater::Function, updater_kwargs::@NamedTuple{}, nsite::Int64, reverse_step::Bool, current_time::Float64, outputlevel::Int64, time_step::Float64, normalize::Bool, direction::Base.Order.ForwardOrdering, noise::Bool, which_decomp::Nothing, svd_alg::Nothing, cutoff::Float64, maxdim::Int64, mindim::Int64, maxtruncerr::Float64) @ ITensorMPS ~/.julia/packages/ITensorMPS/hghpU/src/solvers/sweep_update.jl:187 [14] sub_sweep_update(direction::Base.Order.ForwardOrdering, reduced_operator::ProjMPO, state::MPS; updater::Function, updater_kwargs::@NamedTuple{}, which_decomp::Nothing, svd_alg::Nothing, sweep::Int64, current_time::Float64, time_step::Float64, nsite::Int64, reverse_step::Bool, normalize::Bool, observer!::ITensorMPS.EmptyObserver, outputlevel::Int64, maxdim::Int64, mindim::Int64, cutoff::Float64, noise::Bool) @ ITensorMPS ~/.julia/packages/ITensorMPS/hghpU/src/solvers/sweep_update.jl:107 [15] sweep_update(order::ITensorMPS.TDVPOrder{2, Base.Order.ForwardOrdering()}, reduced_operator::ProjMPO, state::MPS; current_time::Float64, time_step::Float64, kwargs::@Kwargs{updater::typeof(ITensorMPS.exponentiate_updater), updater_kwargs::@NamedTuple{}, nsite::Int64, reverse_step::Bool, sweep::Int64, observer!::ITensorMPS.EmptyObserver, normalize::Bool, outputlevel::Int64, maxdim::Int64, mindim::Int64, cutoff::Float64, noise::Bool}) @ ITensorMPS ~/.julia/packages/ITensorMPS/hghpU/src/solvers/sweep_update.jl:25 [16] macro expansion @ ~/.julia/packages/ITensorMPS/hghpU/src/solvers/alternating_update.jl:69 [inlined] [17] macro expansion @ ./timing.jl:421 [inlined] [18] alternating_update(operator::MPO, init::MPS; updater::Function, updater_kwargs::@NamedTuple{}, nsweeps::Int64, checkdone::Returns{Bool}, write_when_maxdim_exceeds::Nothing, nsite::Int64, reverse_step::Bool, time_start::Float64, time_step::Float64, order::Int64, observer!::ITensorMPS.EmptyObserver, sweep_observer!::ITensorMPS.EmptyObserver, outputlevel::Int64, normalize::Bool, maxdim::Vector{Int64}, mindim::Int64, cutoff::Vector{Float64}, noise::Bool) @ ITensorMPS ~/.julia/packages/ITensorMPS/hghpU/src/solvers/alternating_update.jl:68 [19] alternating_update @ ~/.julia/packages/ITensorMPS/hghpU/src/solvers/alternating_update.jl:23 [inlined] [20] #tdvp#767 @ ~/.julia/packages/ITensorMPS/hghpU/src/solvers/tdvp.jl:86 [inlined] [21] top-level scope ``` Thanks in advance! ---
**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:** [May 4, 2025, 11:10am UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/2 "2025-05-04T11:10:32Z")
You should change `psi[n] = psi[n] * cdag` to `psi[n] = apply(cdag, psi[n])`. `cdag` is an operator with a pair of primed and unprimed indices, so when you contract it with `psi[n]`the result is a tensor where the site index is primed, and therefore isn’t in the proper form when you put it into `tdvp`. `apply` makes sure that the site index gets unprimed. ---
**Author:** ![Zhen\_H](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/zhen_h/32/764_2.png) [@Zhen\_H](https://itensor.discourse.group/u/Zhen_H)\ **Post date:** [May 4, 2025, 5:15pm UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/3 "2025-05-04T17:15:34Z")
Thanks Matthew! That seems to be the problem. ---
**Author:** ![miles](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/miles/32/6_2.png) [@miles](https://itensor.discourse.group/u/miles)\ **Post date:** [May 4, 2025, 5:38pm UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/4 "2025-05-04T17:38:28Z")
Also, just since I see you are working with fermions, make sure when you apply Cdag or C to a site of an MPS that unless it’s the first site, you also apply the Jordan-Wigner string “F” operator to all previous sites. Otherwise you will get incorrect results. ---
**Author:** ![Zhen\_H](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/zhen_h/32/764_2.png) [@Zhen\_H](https://itensor.discourse.group/u/Zhen_H)\ **Post date:** [May 4, 2025, 6:08pm UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/5 "2025-05-04T18:08:31Z")
Thanks Miles, great point! Just to make sure I understand correctly, when constructing MPOs using `H = MPO(os, sites)` where `sites` are fermions, this Jordan-Wigner business is taken care of automatically, right? ---
**Author:** ![miles](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/miles/32/6_2.png) [@miles](https://itensor.discourse.group/u/miles)\ **Post date:** [May 4, 2025, 6:34pm UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/6 "2025-05-04T18:34:44Z")
Yes, that is correct. But not when using individual operators outside an MPO. ---
**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/itensor/original/1X/d3072b13e047cd06df7f3981547c0917d940dfa4.png) [@system](https://itensor.discourse.group/u/system)\ **Post date:** [May 14, 2025, 6:35pm UTC](https://itensor.discourse.group/t/the-projmpo-itensor-product-error-message-inside-tdvp/2368/7 "2025-05-14T18:35:35Z")
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