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Ab initio density functional theory applied to quasidegenerate problems.

Ireneusz Grabowski1, Victor Lotrich, Rodney J Bartlett

  • 1Institute of Physics, Nicolaus Copernicus University, 87-100 Toruń, Poland. ig@fizyka.umk.pl

The Journal of Chemical Physics
|October 24, 2007
PubMed
Summary
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A new coupled-cluster perturbation theory (CCPT) method enhances ab initio density functional theory (DFT) calculations. This approach improves convergence for quasidegenerate systems, offering more accurate correlation potentials and energies.

Area of Science:

  • Quantum Chemistry
  • Computational Physics
  • Electronic Structure Theory

Background:

  • Ab initio density functional theory (DFT) is a powerful tool for electronic structure calculations.
  • Traditional DFT methods often struggle with quasidegenerate systems.
  • Second-order many-body perturbation theory (MBPT2) has limitations in accuracy and convergence for these systems.

Purpose of the Study:

  • To generalize ab initio DFT to include infinite-order effects using coupled-cluster perturbation theory (CCPT).
  • To improve the accuracy and convergence of correlation potentials and energies for quasidegenerate systems.
  • To investigate the role of single excitations in ab initio DFT calculations.

Main Methods:

  • Development and application of a new coupled-cluster perturbation theory (CCPT2) variant within ab initio DFT.

Related Experiment Videos

  • Redefinition of the unperturbed Hamiltonian to correspond to the CCPT2 orbital dependent functional.
  • Application to Be-isoelectronic systems as a test case for quasidegenerate systems.

    • Main Results:

    • The CCPT2 variant demonstrates superior convergence towards the exact quantum Monte Carlo correlation potential for Beryllium compared to previous methods.
    • The semicanonical choice of the unperturbed Hamiltonian is crucial for MBPT2 convergence, unlike the divergent Kohn-Sham choice.
    • Infinite-order effects from CCPT2 are necessary for achieving accurate potentials and energies, surpassing MBPT2 limitations.

    Conclusions:

    • CCPT2 offers a significant advancement for ab initio DFT, particularly for quasidegenerate systems.
    • The infinite-order summation of single excitations is essential for the convergence of ab initio DFT.
    • This generalized approach provides more accurate electronic structure information than traditional methods.