exp() function in trotter gate creation giving errors

ITensor Julia Questions
1

Hi there, I am trying to make trotter gates to calculate the dynamical structure factor for a model, and have encountered an error when I needed to exponentiate the terms to be included in the gates.

The error I got is for the line

Gj = exp(-im * tau / 2 * hj)

which I got from following the MPS time evolution tutorial for julia. The error I got was:

ERROR: LoadError: MethodError: no method matching exp(::NDTensors.DenseTensor{ComplexF64, 0, Tuple{}, NDTensors.Dense{ComplexF64, Vector{ComplexF64}}})

Closest candidates are:
  exp(::NDTensors.DenseTensor{ElT, N}, ::Tuple{Vararg{Int64, NL}}, ::Tuple{Vararg{Int64, NR}}; ishermitian) where {ElT, N, NL, NR}
   @ NDTensors C:\Users\.julia\packages\NDTensors\vDdB4\src\dense\linearalgebra\decompositions.jl:84
  exp(::ComplexF16)
   @ Base math.jl:1564
  exp(::Zeros.Zero)
   @ Zeros C:\Users\.julia\packages\Zeros\eEqdd\src\Zeros.jl:133
  ...

Stacktrace:
 [1] exp(A::ITensor, Linds::Tuple{}, Rinds::Tuple{}; ishermitian::Bool)
   @ ITensors C:\Users\.julia\packages\ITensors\iifFp\src\tensor_operations\matrix_algebra.jl:79
 [2] exp(A::ITensor, Linds::Tuple{}, Rinds::Tuple{})
   @ ITensors C:\Users\.julia\packages\ITensors\iifFp\src\tensor_operations\matrix_algebra.jl:39
 [3] exp(A::ITensor; kwargs::@Kwargs{})
   @ ITensors C:\Users\.julia\packages\ITensors\iifFp\src\tensor_operations\matrix_algebra.jl:86
 [4] exp(A::ITensor)
   @ ITensors C:\Users\.julia\packages\ITensors\iifFp\src\tensor_operations\matrix_algebra.jl:83
 [5] dsfOp(model_params::@NamedTuple{…}, sites::Vector{…}, tau::Float64)
   @ Main C:\Users\Desktop\PO_stuff\current_build\test\julia\site_user.jl:208
 [6] top-level scope
   @ C:\Users\Desktop\PO_stuff\current_build\test\julia\run.jl:120
 [7] include(fname::String)
   @ Base.MainInclude .\client.jl:489
 [8] top-level scope
   @ REPL[8]:1
in expression starting at C:\Users\Desktop\PO_stuff\current_build\test\julia\run.jl:6
Some type information was truncated. Use `show(err)` to see complete types.

Please help me and thanks!

2

Can you please provide a minimal working example? What are tau and hj?
It appears hj may be constructed incorrectly

3

Hi Ryan, thanks for the reply. You are right, I just found the bug to be an incorrect hj construction. I was applying both the Sz and identity on the same site. The code is working now thanks

4

Weird that it isn’t working. It looks like you are trying to take the exponential of a scalar because the the function call above says that there are no indices and has an order of 0. I was able to replicate the problem like this

A = ITensor(NDTensors.Dense(ComplexF32, undef, 1), ())
fill!(A, ComplexF32(1.0))
exp(NDTensors.tensor(A))

And you can solve the problem by extracting the data from the tensor like this A[]
Can you please let me know what version of ITensors.jl you are using becuase I don’t see the file src\tensor_operations\matrix_algebra.jl in the latest release. Thanks!