Efficient multireference perturbation theory without high-order reduced density matrices

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Strongly contracted n-electron valence state perturbation theory (SC-NEVPT) is a powerful quantum chemistry method.
• Accurate calculations of electronic structures are crucial for understanding molecular properties.
• Existing methods may involve approximations or computational limitations.

Purpose of the Study:

• To introduce a novel stochastic approach for SC-NEVPT calculations.
• To perform SC-NEVPT2 calculations without approximations.
• To assess the accuracy and scalability of the developed method.

Main Methods:

• Developed a stochastic algorithm for SC-NEVPT.
• Utilized one- and two-body reduced density matrices.
• Applied the method to wave functions from selected configuration interaction and variational Monte Carlo.
• Performed SC-NEVPT2 calculations.

Main Results:

• The stochastic SC-NEVPT approach requires only one- and two-body reduced density matrices.
• Demonstrated accuracy for small test systems.
• Investigated computational scaling with virtual orbitals and molecule size.
• Found SC-NEVPT2 energy to be insensitive to reference wave function quality.
• Applied the method to Fe(II)-porphyrin and [Cu2O2]2+ systems.

Conclusions:

• The stochastic SC-NEVPT method provides an accurate and efficient way to compute electronic structures.
• The approach is robust and applicable to various wave function types.
• This method advances the capabilities for studying complex chemical systems.