Going beyond "no-pair relativistic quantum chemistry".

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Relativistic quantum chemistry (RQC) traditionally relies on approximations like the no-pair approximation (NPA).
• Existing methods to remove the NPA have fundamental defects or remain incomplete.
• Retardation effects, crucial for accuracy, are often excluded from RQC.

Purpose of the Study:

• To propose a novel effective many-body (EMB) QED approach for relativistic quantum chemistry.
• To resolve inherent defects within the no-pair approximation (NPA).
• To develop a method consistent with standard electronic structure methodologies.

Main Methods:

• Developed an effective many-body (EMB) QED approach.
• Derived the second-order energy (E2) for many-electron systems using both algebraic and diagrammatic methods.
• Introduced a potential-independent no-pair approximation (PI-NPA) by incorporating QED counter terms.

Main Results:

• The EMB-QED approach is shown to be in full accordance with standard electronic structure methods.
• The second-order energy (E2) was derived, equivalent to using 3 Goldstone-like diagrams or 28 Feynman diagrams.
• The potential dependence of the NPA was removed, leading to the PI-NPA.

Conclusions:

• The proposed EMB-QED approach offers a robust framework for relativistic molecular quantum mechanics.
• The development of PI-NPA provides a more accurate and reliable alternative to the traditional NPA.
• This work establishes a continuous spectrum from NPA to full QED, enhancing theoretical capabilities.