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implement truncation_error keyword arg for truncate! #99

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implement truncation_error keyword arg for truncate! #99

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5c887ce
implement truncation_error keyword arg for truncate!
NuclearPowerNerd Nov 20, 2024
00f1764
Refactor truncate! to return truncation error from each bond
NuclearPowerNerd Jan 2, 2025
4a29839
Apply suggestions from code review
NuclearPowerNerd Jan 13, 2025
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14 changes: 11 additions & 3 deletions src/abstractmps.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1672,13 +1672,18 @@ provided as keyword arguments.

Keyword arguments:
* `site_range`=1:N - only truncate the MPS bonds between these sites
* `truncation_errors` - if provided, will store the truncation error from each SVD performed in a single call to `truncate!`. This should be a `Ref` type, for example `truncation_errors = Ref{Vector{Float64}}()`. It should be initialized to some value (likely 0.0, e.g., `truncation_errors[] = zeros(nbonds)`).
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"""
function truncate!(M::AbstractMPS; alg="frobenius", kwargs...)
return truncate!(Algorithm(alg), M; kwargs...)
end

function truncate!(
::Algorithm"frobenius", M::AbstractMPS; site_range=1:length(M), kwargs...
::Algorithm"frobenius",
M::AbstractMPS;
site_range=1:length(M),
truncation_errors=nothing,
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kwargs...,
)
N = length(M)

Expand All @@ -1687,10 +1692,13 @@ function truncate!(
orthogonalize!(M, last(site_range))

# Perform truncations in a right-to-left sweep
for j in reverse((first(site_range) + 1):last(site_range))
for (i,j) in enumerate(reverse((first(site_range) + 1):last(site_range)))
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rinds = uniqueinds(M[j], M[j - 1])
ltags = tags(commonind(M[j], M[j - 1]))
U, S, V = svd(M[j], rinds; lefttags=ltags, kwargs...)
U, S, V, spec = svd(M[j], rinds; lefttags=ltags, kwargs...)
if !isnothing(truncation_errors)
truncation_errors[][i] = spec.truncerr
end
M[j] = U
M[j - 1] *= (S * V)
setrightlim!(M, j)
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12 changes: 12 additions & 0 deletions test/base/test_mps.jl
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Expand Up @@ -755,6 +755,18 @@ end
truncate!(M; site_range=3:7, maxdim=2)
@test linkdims(M) == [2, 4, 2, 2, 2, 2, 8, 4, 2]
end

@testset "truncate! with truncation_errors" begin
N = 10
nbonds = N - 1
M = basicRandomMPS(10; dim=10)
truncation_errors = Ref{Vector{Float64}}()
truncation_errors[] = fill(-1.0, nbonds) # set to something other than zero for test.
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@mtfishman mtfishman Jan 13, 2025 •
edited
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It would be better to design the code so that users don't have to initialize the list ahead of time. We could add a line:

if isnothing(truncation_errors!)
  truncation_errors![] = Vector{real(scalartype(M))}(undef, length(M) - 1)
end

at the top of truncate!(...) (note the change to truncation_errors! based on my comment above).

truncate!(M, maxdim=3, cutoff=1E-3, truncation_errors=truncation_errors)
@test all(truncation_errors[] .>= 0.0)
end


end

@testset "Other MPS methods" begin
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