DMRG calculations are staggeringly slower than in previous versions #381

# Parameters for the Hamiltonian
L = 30
χ = 1
Δ = 0.1
Ω = 0.0
C = 1.0

function potential_vdW(L, C)
    f(i, j) = i==j ? 0 : abs(i-j)^(-6)
    V = [C*f(i,j) for i in 1:L, j in 1:L]
    return V
end

function rydberg_chain_hamiltonian(L, Ω, Δ, V)
    n̂ = TensorMap(ComplexF64[0 0; 0 1], ℂ^2 → ℂ^2)
    chain = fill(ℂ^2, L)
    single_site = Dict()
    for I in eachindex(chain)
        single_site[(I,)] = (Ω/2)*σˣ() - Δ*n̂
    end
    two_site = Dict()
    for I in 1:L 
        for J in I+1:L 
            two_site[(I,J)] = V[I,J]* (n̂⊗n̂)
        end
    end

    H = FiniteMPOHamiltonian(chain, single_site)+FiniteMPOHamiltonian(chain, two_site)

    return H
end 

H = rydberg_chain_hamiltonian(L, Ω, Δ, V)
ψ₀ = FiniteMPS(rand, ComplexF64, L, ℂ^2, ℂ^χ)
println("Starting ground state calculation.")
@btime ψ, _, _ = find_groundstate(ψ₀, H, DMRG(; tol=1e-6, verbosity=0))