tdvp time developing

ITensor Julia Questions
,
1

Hey all!
Can someone explain me we I can’t use tdvp method on the randomMPS function ?

If you are including code in your question or comment, please put it in a code block as follows:

using ITensors
ψ= randomMPS(site)
ψ= tdvp(H,ψ,-1.0*δβ)

Does some know what is the problem ?
The error is:

InexactError: Float64(1.1312004476239226 + 1.3877787807814457e-16im)

2

You can use TDVP on a random MPS, so the error must be coming from somewhere else. Could you provide more context? What is the full stack trace of the error message?

3

yes sure,
This is the error I got

ERROR: InexactError: Float64(1.1312004476239226 + 1.3877787807814457e-16im)
Stacktrace:
[1] Real
@ ./complex.jl:44 [inlined]
[2] convert
@ ./number.jl:7 [inlined]
[3] setindex!(::Matrix{Float64}, ::ComplexF64, ::Int64, ::Int64)
@ Base ./array.jl:905
[4] applyexp(H::ProjMPO, tau::Float64, x0::ITensor; kwargs::Base.Pairs{Symbol, Real, NTuple{4, Symbol}, NamedTuple{(:tol, :maxiter, :outputlevel, :current_time), Tuple{Float64, Int64, Int64, Float64}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/applyexp.jl:53
[5] (::ITensorTDVP.var"#solver#38"{ITensorTDVP.var"#solver#37#39"{Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}}})(H::ProjMPO, t::Float64, psi0::ITensor; kws::Base.Pairs{Symbol, Real, Tuple{Symbol, Symbol}, NamedTuple{(:current_time, :outputlevel), Tuple{Float64, Int64}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp.jl:27
[6] tdvp_site_update!(nsite_val::Val{2}, reverse_step_val::Val{true}, solver::ITensorTDVP.var"#solver#38"{ITensorTDVP.var"#solver#37#39"{Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}}}, PH::ProjMPO, psi::MPS, b::Int64; current_time::Float64, outputlevel::Int64, time_step::Float64, normalize::Bool, direction::Base.Order.ForwardOrdering, noise::Float64, which_decomp::Nothing, svd_alg::String, cutoff::Float64, maxdim::Int64, mindim::Int64, maxtruncerr::Float64)
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_step.jl:353
[7] tdvp_site_update!(solver::ITensorTDVP.var"#solver#38"{ITensorTDVP.var"#solver#37#39"{Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}}}, PH::ProjMPO, psi::MPS, b::Int64; nsite::Int64, reverse_step::Bool, current_time::Float64, outputlevel::Int64, time_step::Float64, normalize::Bool, direction::Base.Order.ForwardOrdering, noise::Float64, which_decomp::Nothing, svd_alg::String, cutoff::Float64, maxdim::Int64, mindim::Int64, maxtruncerr::Float64)
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_step.jl:154
[8] tdvp_sweep(direction::Base.Order.ForwardOrdering, solver::Function, PH::ProjMPO, time_step::Float64, psi::MPS; kwargs::Base.Pairs{Symbol, Real, NTuple{7, Symbol}, NamedTuple{(:current_time, :reverse_step, :sweep, :maxdim, :mindim, :cutoff, :noise), Tuple{Float64, Bool, Int64, Int64, Int64, Float64, Float64}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_step.jl:78
[9] tdvp_step(order::ITensorTDVP.TDVPOrder{2, Base.Order.ForwardOrdering()}, solver::Function, PH::ProjMPO, time_step::Float64, psi::MPS; current_time::Float64, kwargs::Base.Pairs{Symbol, Real, NTuple{6, Symbol}, NamedTuple{(:reverse_step, :sweep, :maxdim, :mindim, :cutoff, :noise), Tuple{Bool, Int64, Int64, Int64, Float64, Float64}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_step.jl:9
[10] macro expansion
@ ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_generic.jl:82 [inlined]
[11] macro expansion
@ ./timing.jl:299 [inlined]
[12] tdvp(solver::Function, PH::ProjMPO, t::Float64, psi0::MPS; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_generic.jl:81
[13] tdvp
@ ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_generic.jl:44 [inlined]
[14] tdvp(solver::Function, H::MPO, t::Float64, psi0::MPS; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_generic.jl:148
[15] tdvp
@ ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp_generic.jl:142 [inlined]
[16] tdvp(H::MPO, t::Float64, psi0::MPS; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp.jl:47
[17] tdvp
@ ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp.jl:47 [inlined]
[18] #tdvp#43
@ ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp.jl:55 [inlined]
[19] tdvp(H::MPO, psi0::MPS, t::Float64)
@ ITensorTDVP ~/.julia/packages/ITensorTDVP/H26Xf/src/tdvp.jl:55
[20] top-level scope
@ REPL[9]:1

4

I don’t know whether this is already solved as I am too late in this thread.
Inexact error generally arise when you are trying to do a type conversion that may require truncation.Here you are trying to convert a complex number to float. In Julia, the complex floats belong to a different data type ComplexF64. From the stacktrace,it seems like the applyexp function is getting Complex value where it expects real float.Does it only appear if you have a random initial state?
Do you get this error for both real and imaginary time evolution?

5

Thanks for replying @sandipanmanna. I think this is a bug with the “applyexp” solver along the lines you are saying.

@tsegev: as a workaround for now, please try passing the following keyword argument into tdvp which is tdvp(...; solver_backend="exponentiate", ...)

The "exponentiate" solver ought to not have the same bug, and I think we will be switching to it as the default in the future anyway.