Matrix Product States with Large Sites.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Matrix Product States (MPS) are powerful tools for simulating quantum many-body systems.
• Traditional clustering in MPS can lead to prohibitive computational costs, limiting applications.
• Describing correlated multireference (MR) electronic structures requires advanced computational methods.

Purpose of the Study:

• To investigate novel strategies for grouping orbitals into clusters within MPS.
• To propose an efficient MPS cluster structure for enhanced computational advantage.
• To apply this formalism to various correlated multireference theories.

Main Methods:

• Exploration of generic versus specialized MPS cluster structures.
• Development of a special cluster MPS involving only the first and last orbitals/sites.
• Implementation of uncontracted multireference configuration interaction, perturbation, and linearized coupled cluster theories using the proposed MPS.

Main Results:

• A special cluster MPS structure offers significant computational advantages over generic approaches.
• The proposed method successfully implements various uncontracted MR theories.
• Applications to nitrogen dimer, chromium dimer, and benzene demonstrate utility with up to two large cluster sites.

Conclusions:

• The special cluster MPS formalism provides a computationally efficient route for multireference electronic structure calculations.
• This approach is flexible and useful for analyzing the accuracy of different multireference approximations.
• The method enables accurate descriptions of complex molecular systems with correlated electronic structures.