Calculation of electronic excitations using wave-function in wave-function frozen-density embedding.

Area of Science:

• Quantum chemistry
• Computational physics
• Molecular modeling

Background:

• A general framework for frozen-density embedding (FDE) was recently established.
• Accurate treatment of complex molecular systems often requires high-level computational methods for all parts.

Purpose of the Study:

• To develop and validate a novel FDE variant combining coupled-cluster (CC) theory for subsystems and density-functional theory (DFT) for interactions.
• To enable balanced and unbiased computational descriptions of complex systems where all components require CC-level accuracy.
• To extend the applicability of high-level CC methods to larger molecular assemblies.

Main Methods:

• Fragmentation of a supermolecule into subsystems.
• Treatment of all subsystems using coupled-cluster (CC) theory.
• Description of subsystem interactions using density-functional theory (DFT).
• This specific variant is termed wave-function theory in wave-function theory FDE or CC in CC FDE.

Main Results:

• Numerical results for hydrogen-bonded complexes with strong interactions are presented.
• The method demonstrates high accuracy for systems with strong intermolecular interactions.
• Accuracy is expected to be even higher for weakly interacting subsystems.

Conclusions:

• The developed CC in CC FDE method provides a balanced and unbiased description of complex molecular systems.
• This approach facilitates the study of properties of larger complexes, such as DNA base pairs, using high-level CC methods.
• The method overcomes limitations of embedding single molecules in solvation shells, focusing instead on the accurate treatment of multiple interacting subsystems.