Configuration Interaction for the Helium Isoelectronic Series

Configuration interaction (CI) provides a systematic method for improving on single-configuration Hartree–Fock (HF) computations [1, 2]. This Demonstration considers the two-electron atoms in the helium isoelectronic series. The HF wavefunction is the optimal product of one-electron orbitals approximating the ground-state configuration. An improved representation of the ground state can be obtained by a superposition containing excited electronic configurations, including , , , …, with relative contributions determined by the variational principle.
Shull and Löwdin [3] represented the total wavefunction in the form
where is the angle between and , while is the Legendre polynomial of degree . Note that the functions contain internal angular dependence but can still represent atomic states, such as and , with . In the computations presented in this Demonstration, we consider only the , , and contributions to CI. The contribution is taken as the HF function , which can be very closely approximated using double-zeta orbitals
The contribution is represented by the orthogonalized Slater-type function
Together, these two contributions can closely approximate the -limit to the CI function, as defined by Shull and Löwdin. The and contributions are represented using simple Slater-type orbitals:
for and
for .
All the relevant matrix elements of the Hamiltonian are then computed; for example,
and so forth. All energies are expressed in Hartree atomic units: .
You can select the level of configuration interaction: , -limit, + or ++, and the values of the exponential parameters , and . The built-in Mathematica function Eigenvalues then finds the lowest eigenvalue for the corresponding Hamiltonian matrix. The results are represented graphically on a barometer display, comparing them to the exact values of the energy.
Plots of the radial distribution function for each component configuration are also shown on the left. The relative magnitudes are not to scale.


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Snapshot 1: the single-configuration Hartree–Fock result
Snapshot 2: approximation to the -limit; a more accurate computation gives hartrees
Snapshot 3: result using optimized parameters; reaching is considered a milestone
[1] S. M. Blinder, "The Hartree–Fock Approximation," Mathematical Physics in Theoretical Chemistry (S. M. Blinder and J. E. House, eds.), Amsterdam: Elsevier, forthcoming.
[2] Wikipedia. "Configuration Interaction." (Feb 14, 2018)
[3] H. Shull and P.-O. Löwdin, "Superposition of Configurations and Natural Spin Orbitals: Applications to the He Problem," Journal of Chemical Physics, 30(3), 1959 pp. 617–626. doi:10.1063/1.1730019.
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