[Paper Review] Fragment-Based Configuration Interaction: Towards a Unifying Description of Biexcitonic Processes in Molecular Aggregates

Johannes E. Adelsperger, Coen de Graaf+1 more|arXiv (Cornell University)|Feb 18, 2026

Luminescence and Fluorescent Materials0 citations

TL;DR

The paper develops a unified ab initio fragment-based configuration-interaction framework (SymbolicCI and NOCI-F) to describe the full biexcitonic manifold (LE, CT, TT, CTX, CTCT) in molecular aggregates and benchmarks their inter-biexciton couplings.

ABSTRACT

Biexcitonic states govern singlet fission, triplet-triplet and exciton-exciton annihilation, yet a unified understanding of how these processes compete within a shared electronic manifold remains elusive. We outline a conceptual framework based on fragment-based configuration-interaction that systematically constructs diabatic Hamiltonians spanning the full one-particle (LE, CT) and two-particle (LELE, CTCT, TT, CTX with X = LE, CT, or T) manifolds from monomer-local building blocks, preserving physical interpretability throughout. SymbolicCI provides analytic Hamiltonian matrix elements for efficient large-scale calculations; NOCI-F delivers benchmark-quality couplings. The resulting diabatic Hamiltonians can be coupled to quantum dynamics simulations. Applications to ethylene aggregates and the anthracene crystal reveal CTX configurations as electronic gateways bridging excitonic manifolds, with CT-mediated relaxation pathways competing with conventional annihilation. In H-type aggregates, LECT admixture stabilizes a "bi-excimer" analogous to one-particle excimers. By providing first-principles access to biexciton formation, separation, and transport, we hope to stimulate further exchange between electronic structure and quantum dynamics communities toward a predictive understanding of multiexcitonic photophysics.

Motivation & Objective

• Motivate the need for a unified, first-principles description of biexcitons spanning one- and two-particle manifolds.
• Introduce fragment-based CI frameworks that preserve physical interpretability while enabling large-scale calculations.
• Provide first-principles access to formation, coupling, and transport pathways of biexcitons in aggregates.
• Demonstrate the framework on representative ethylene and anthracene systems to reveal connectivity and packing effects.

Proposed method

• Introduce SymbolicCI: an orbital-based CI approach with fragment-local active spaces that yields analytic, spin-adapted Hamiltonians spanning LE, CT, triplet, and biexcitonic configurations.
• Provide analytic expressions for Hamiltonian matrix elements in terms of one- and two-electron integrals, enabling interpretability of couplings.
• Introduce NOCI-F: a nonorthogonal CI method using fully optimized fragment states (multiconfigurational) to benchmark biexciton couplings with orbital relaxation and dynamic correlation.
• Compare SymbolicCI and NOCI-F on identical subspaces to validate representational consistency and explore inter-manifold couplings.
• Apply the methods to ethylene aggregates (15-mers) and anthracene pentamers/crystal to analyze CTX roles and packing-dependent biexciton behavior.

Experimental results

Research questions

• RQ1 How can one construct a unified ab initio Hamiltonian that includes all relevant one- and two-particle biexciton configurations from chemically meaningful fragment building blocks?
• RQ2 What are the dominant couplings and pathways connecting LE, CT, TT, and CTX configurations in extended aggregates, and how do packing and charge-transfer admixture influence them?
• RQ3 Can SymbolicCI and NOCI-F provide consistent, interpretable descriptions of biexciton manifolds and accurate inter-biexciton couplings for large systems?
• RQ4 How do CTX configurations function as electronic gateways between excitonic manifolds, and what implications do they have for relaxation pathways and transport?
• RQ5 What insights do these frameworks offer for the design of materials with predictable multiexcitonic photophysics?

Key findings

• Fragment-based CI provides a chemically interpretable, first-principles route to generate the full biexcitonic manifold from monomer-local building blocks.
• SymbolicCI yields analytic, spin-adapted Hamiltonians with explicit expressions for biexcitonic couplings, clarifying how topology and locality affect diffusion and exchange pathways.
• NOCI-F delivers benchmark-quality couplings by combining fully relaxed fragment states within a nonorthogonal CI framework, enabling validation against SymbolicCI.
• Applications to ethylene 15-mers and anthracene systems reveal CTX configurations as electronic gateways bridging excitonic manifolds and competing with conventional annihilation pathways.
• In H-type aggregates, LECT admixture stabilizes a bi-excimer-like feature, highlighting complex inter-manifold mixing governed by packing and CT admixtures.
• The frameworks enable first-principles access to biexciton formation, diffusion, and transport, offering a pathway to predictive multiexcitonic photophysics.