Approximation to Second Order N-Electron Valence State Perturbation Theory: Limiting the Wave Function within...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Second-order N-electron valence state perturbation theory (NEVPT2) is a robust method for electronic structure calculations.
• Linearized adiabatic connection (AC0) theory offers a computationally less demanding approximation.
• Calculating high-order reduced density matrices (RDMs) presents a significant computational challenge in NEVPT2.

Purpose of the Study:

• To develop a novel approximation to NEVPT2 that reduces computational cost.
• To evaluate the performance of the new method, NEVPTS, against established methods like NEVPT2 and AC0.
• To assess the accuracy of NEVPTS for various chemical applications, including potential energy curves and excitation energies.

Main Methods:

• Development of NEVPTS, an approximation utilizing single-excitation wave function amplitudes.
• NEVPTS requires only 3rd-order RDMs, circumventing the need for 4th-order RDMs.
• Comparative studies with NEVPT2 and AC0 on diatomic molecules, biradicals, and organic molecules.

Main Results:

• NEVPTS shows comparable accuracy to NEVPT2 for potential energy curves of diatomic molecules and singlet-triplet gaps in biradicals.
• NEVPTS significantly outperforms AC0 in these specific applications.
• For excitation energies of organic molecules, NEVPTS is less accurate than NEVPT2.
• The computational cost and performance of NEVPTS fall between those of NEVPT2 and AC0.

Conclusions:

• NEVPTS presents a computationally efficient alternative to NEVPT2 for specific electronic structure problems.
• The method offers a practical trade-off between computational expense and accuracy.
• NEVPTS is a valuable addition to the toolkit for electronic structure calculations, particularly when computational resources are a concern.