"""
Computes the Linearized CCD AKA CEPA(0) without singles, correlation energy.
Equations taken by linearizing Eq. 153 of [Crawford:2000:33].

__authors__   =  "Jonathon P. Misiewicz"
__credits__   =  ["Jonathon P. Misiewicz"]

__copyright__ = "(c) 2014-2020, The Psi4NumPy Developers"
__license__   = "BSD-3-Clause"
"""

import numpy as np
import psi4
from DSD import DirectSumDiis
from integrals import integrals

### Settings

mol = psi4.geometry("""
O
H 1 1.0
H 1 1.0 2 104.5
symmetry c1""")
scf_type = "pk"
target_convergence = 7
freeze_core = True
maxiter = 50
compare_psi4 = True
basis = "cc-pvdz"

### Setup
psi4.set_options({"freeze_core": freeze_core, "scf_type": scf_type, "e_convergence": target_convergence + 1, "basis": basis})
I, F = integrals(mol)
t2 = np.zeros(I["oovv"].shape)
Fo = F["oo"].diagonal()
Fv = F["vv"].diagonal()
D = Fo.reshape(-1, 1, 1, 1) + Fo.reshape(-1, 1, 1) - Fv.reshape(-1, 1) - Fv
dsd = DirectSumDiis(3, 8)

### Main Loop
for i in range(1, maxiter + 1):
    # Two Electron Terms
    r2 = I["oovv"] + 0.5 * np.einsum("ABcd, IJcd -> IJAB", I["vvvv"], t2, optimize=True)
    r2 += 0.5 * np.einsum("klIJ, klAB -> IJAB", I["oooo"], t2, optimize=True)
    temp = np.einsum("AkIc, JkBc -> IJAB", I["voov"], t2, optimize=True)
    r2 += temp + temp.transpose((1, 0, 3, 2)) - temp.transpose((0, 1, 3, 2)) - temp.transpose((1, 0, 2, 3))
    # One Electron Terms. For canonical orbitals, this will reduce to -t2, after dividing by D
    temp = - np.einsum("Ii, iJAB -> IJAB", F["oo"], t2, optimize=True)
    r2 += temp - temp.transpose((1, 0, 2, 3))
    temp = + np.einsum("aA, IJaB -> IJAB", F["vv"], t2, optimize=True)
    r2 += temp - temp.transpose((0, 1, 3, 2))
    # Step
    t2 += r2 / D
    t2 = dsd.diis(r2, t2)
    r_norm = np.linalg.norm(r2)
    Elccd = 0.25 * np.sum(I["oovv"] * t2)
    print(f"{i:3d} E={Elccd:3.10f} R = {r_norm:0.8f}")
    if r_norm < float(f"1e-{target_convergence}"):
        break
else:
    raise Exception("Equations did not converge.")

if compare_psi4:
   wfn = psi4.energy("lccd", return_wfn=True)[1]
   psi4.compare_values(wfn.variable("CURRENT CORRELATION ENERGY"), Elccd, target_convergence, "LCCD Energy")

@@ -2083,19 +2083,20 @@
 _values(
-wfn
+psi4
 .variabl

Update LCCD.py