Calculating Molecular Polarizabilities Using Exact Frozen Density Embedding with External Orthogonality

Area of Science:

• Quantum Chemistry
• Computational Materials Science
• Electronic Structure Theory

Background:

• Frozen density embedding (FDE) is a formally exact quantum mechanical embedding scheme.
• Practical FDE implementations face limitations with overlapping subsystems due to approximate functionals.
• Enforcing external orthogonality (EO) circumvents the need for approximate functionals, enabling exact FDE for strongly overlapping systems.

Purpose of the Study:

• To implement exact coupled FDE with EO (FDEc-EO) for polarizability calculations.
• To validate the method against supermolecular time-dependent density functional theory (TDDFT) results.
• To analyze the implications of nonunique density partitioning on subsystem polarizabilities.

Main Methods:

• Implementation of exact FDEc-EO within the Amsterdam Density Functional (ADF) program package.
• Enforcement of EO using the level-shift projection operator method for orthogonal fragment orbitals.
• Focus on symmetric EO contributions to the induced density matrix for pure functionals.

Main Results:

• The FDEc-EO implementation accurately reproduces supermolecular TDDFT polarizabilities.
• Subsystem polarizability interpretation is limited by nonunique density partitioning, dependent on initial polarization.
• Localized subsystem excitation energies are stable, suggesting minimization of nonuniqueness through fragmentation.

Conclusions:

• Exact FDEc-EO provides a robust method for calculating accurate polarizabilities, overcoming previous limitations.
• The nonunique partitioning of density affects global properties like polarizability, irrespective of fragmentation.
• Minimizing nonuniqueness for localized properties is achievable via careful system fragmentation.