The 4-index 2-electron integral can be obtained for the He atom with the following code:
import numpy as np
from pyscf import gto, scf, ao2mo
mol = gto.Mole()
mol.atom = """
He 0. 0. 0.
"""
mol.basis = "cc-pvdz"
mol.build()
# Run Hartree-Fock.
mf = scf.RHF(mol)
mf.kernel()
# Find electron-repulsion integrals (eri).
eri = ao2mo.kernel(mol, mf.mo_coeff)
eri = np.asarray(ao2mo.restore(1, eri, mol.nao))
I am interested in obtaining the 2-index (A) and 3-index (CIAB) matrices, which can be used to form the 4-index 2-electron (eri) integral. I have looked at the list of possible means by which this is accomplished in PySCF code; however, I am unsure of the correct answer.