Bootstrap embedding with an unrestricted mean-field bath

Area of Science:

• Quantum chemistry
• Computational physics
• Electronic structure theory

Background:

• Quantum embedding methods utilize Schmidt decomposition of the full system wavefunction to preserve entanglement between system fragments and the bath.
• The accuracy of these methods is sensitive to the quality of the initial full system wavefunction.
• Current methods like bootstrap embedding (BE) typically start from restricted mean-field wavefunctions (RBE).

Purpose of the Study:

• To develop and apply an unrestricted Hartree-Fock (HF) based bootstrap embedding (UBE) method.
• To improve quantum embedding for strongly correlated systems by leveraging spin-unrestricted wavefunctions.
• To assess UBE's performance in modeling hydrogen ring systems and calculating electronic properties.

Main Methods:

• Generalization of bootstrap embedding (BE) to an unrestricted Hartree-Fock (HF) bath, termed unrestricted BE (UBE).
• Application of UBE to model hydrogen ring systems, including high spin symmetry states.
• Computation of ionization potentials, electron affinities, and spin-splittings using UBE.

Main Results:

• UBE successfully models high spin symmetry states, overcoming limitations of RBE.
• UBE achieves accuracy comparable to spin-unrestricted coupled cluster singles and doubles for computed electronic properties.
• UBE demonstrates more reliable convergence compared to RBE, even in cases where RBE is applicable.

Conclusions:

• Unrestricted bootstrap embedding (UBE) offers significant improvements over RBE for strongly correlated systems.
• UBE enhances the capability to study systems with high spin symmetry.
• Future improvements to RBE and density matrix embedding theory can also benefit UBE.