Multi-reference perturbation theories based on the DOCI wavefunction.

Area of Science:

• Quantum Chemistry
• Computational Physics

Background:

• Full configuration interaction (FCI) calculations are computationally expensive.
• The doubly occupied configuration interaction (DOCI) wavefunction provides a starting point for approximations.
• Seniority-number basis offers an alternative framework for electronic structure calculations.

Purpose of the Study:

• To develop an alternative approach for accessing FCI energy.
• To utilize perturbation expansions within the seniority-number basis.
• To validate the accuracy of the new method against FCI energy.

Main Methods:

• Derivation of Rayleigh-Schrödinger perturbation theories up to fourth-order from the DOCI ground state.
• Development of second-order perturbation theory using the DOCI-Fock operator as the zeroth-order Hamiltonian.
• Application of the DOCI-SCF-RSPT4 and DOCI-SCF-MP2 methods to small molecules.

Main Results:

• The DOCI-SCF-RSPT4 and DOCI-SCF-MP2 methods quantitatively reproduced the FCI energy for small molecules.
• Demonstrative calculations confirmed the accuracy of the developed perturbation theories.
• The study highlights the potential of the seniority-number basis for accurate electronic structure calculations.

Conclusions:

• The developed perturbation expansion method provides an accurate alternative to FCI energy calculations.
• Connections to density matrix renormalization group and quantum computing may enable solutions for complex many-electron problems.
• This work advances computational methods in quantum chemistry for solving challenging electronic structure problems.