From 99be9bdb78a32c0e45430cf0e5c8ca05dd33111e Mon Sep 17 00:00:00 2001
From: Nuwan Yapa <yapanuwan@gmail.com>
Date: Thu, 4 Jun 2026 21:30:38 -0400
Subject: [PATCH] EC+ACCC implemented (works well, but not amazing)

---
 calculations/EC+ACCC.jl | 74 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 calculations/EC+ACCC.jl

diff --git a/calculations/EC+ACCC.jl b/calculations/EC+ACCC.jl
new file mode 100644
index 0000000..34e43e1
--- /dev/null
+++ b/calculations/EC+ACCC.jl
@@ -0,0 +1,74 @@
+using Roots, LinearAlgebra, Plots
+
+include("../ho_basis.jl")
+include("../EC.jl")
+include("../common.jl")
+
+V_of_r(r) = 2 * exp(-(r-3)^2 / (1.5)^2)
+Λ = 0
+m = 1.0
+
+ϕ = 0.0
+μω_global = 0.5 * exp(-2im * ϕ)
+E_max = 40
+
+H0 = get_3b_H_matrix(jacobi, V_of_r, μω_global, E_max, Λ, m, true, true)
+
+# Vp = perturbation to make the state artificially bound
+Vp_of_r(r) = -exp(-(r/3)^2)
+@time "Vp" Vp = get_3b_H_matrix(jacobi, Vp_of_r, μω_global, E_max, Λ, m, false, true)
+
+ref_E = -2.22
+training_c = [3.0, 2.6, 2.2, 1.8]
+
+EC = affine_EC(H0, Vp)
+train!(EC, training_c; ref_eval=ref_E, CAEC=false)
+training_E = EC.training_E
+
+quick_extrapolate(c) = argmin(real, get_extrapolated_evals(EC.H0_EC, EC.H1_EC, EC.N_EC, c, 0))
+
+n_EC = 10
+EC_c = (1.8 .+ rand(n_EC)) .+ 0.001im .* (-2 .+ 4 * rand(n_EC))
+
+target_c = 0.0 : 0.2 : 1.2
+exact_E = [4.076662025307587-0.012709842443350328im,
+    3.613318119833891-0.007335804709990623im,
+    3.1453431847006783-0.004030580410326795im,
+    2.672967129943755-0.00211498327461944im,
+    2.196542557810288-0.0010719835443437104im,
+    1.7164583929199813-0.0005455212208182736im,
+    1.233088227541505-0.0003070320106485624im]
+
+EC_E = [quick_extrapolate(c) for c in EC_c]
+
+# determining c0 with EC
+c0 = find_zero(c -> abs2(quick_extrapolate(c)), 0.85)
+println("Estimated c0 = ", c0)
+
+EC_k = alt_sqrt.(EC_E)
+
+order::Int = ceil((length(EC_c) - 1) / 2) # order of the Pade approximant
+
+# Solve coefficients as a linear system
+M_left_element(c, i) = alt_sqrt(c - c0)^i
+M_left = M_left_element.(EC_c, (0:order)')
+M_right = -EC_k .* M_left[:, 2:end] # remove the first column
+M = hcat(M_left, M_right) # M = [M_left | M_right]
+sol = M \ EC_k
+a = sol[1:order+1]
+b = [1; sol[order+2:end]]
+
+# Pade approximant
+polynomial(a, c) = sum(i -> a[i+1] * alt_sqrt(c - c0)^i, 0:order)
+pade_approx(c) = polynomial(a, c) / polynomial(b, c)
+
+# Extrapolate
+extrapolated_k = pade_approx.([EC_c; target_c])
+extrapolated_E = (extrapolated_k .^ 2)
+
+# Plotting
+scatter(real.(training_E), imag.(training_E), label="training")
+scatter!(real.(exact_E), imag.(exact_E), label="exact")
+scatter!(real.(EC_E), imag.(EC_E), label="EC", m=:star5)
+scatter!(real.(extrapolated_E), imag.(extrapolated_E), label="ACCC", m=:x)
+savefig("temp/EC+ACCC.pdf")