Accurate and systematically improvable density functional theory embedding for correlated wavefunctions.

Area of Science:

• Quantum chemistry
• Computational physics

Background:

• Quantum embedding calculations combine wavefunction methods for active subsystems with density functional theory for the remainder.
• Identifying and mitigating error sources is crucial for the accuracy of these hybrid methods.

Purpose of the Study:

• To analyze the sources of error in quantum embedding calculations.
• To develop and validate a corrected embedding scheme for improved accuracy.

Main Methods:

• Projector-based embedding using localized occupied orbitals to partition the system.
• Testing an MP2 correction for the nonadditive exchange-correlation energy term.
• Analyzing discontinuities arising from localized orbitals and their dependence on active region size.

Main Results:

• The error in the nonadditive exchange-correlation energy significantly dominates over the embedding potential's contribution.
• The MP2-corrected embedding scheme accurately reproduces full wavefunction calculations for chemical reactions.
• Discontinuities in embedded energy are small and reducible by increasing the active region size.
• Rapid convergence of reaction energies is observed when the environment's polarization is captured by DFT.

Conclusions:

• The nonadditive exchange-correlation energy is the primary error source in quantum embedding calculations.
• The MP2-corrected embedding scheme offers a reliable approach for accurate quantum embedding.
• The method demonstrates good convergence and robustness even for complex systems like conjugated molecules.