This repository contains the computational framework, simulation code, and raw data used in the manuscript "Quantum Entanglement and Information Theory in Radical Pair Navigation." We built the simulation framework using Python and the Quantum Toolbox in Python (QuTiP v4.7). The code models the open quantum system dynamics of the FAD•−/Trp•+ radical pair mechanism (RPM) implicated in avian magnetoreception. The repository includes scripts to compute the following: Spin Hamiltonian Dynamics: Construction of the two-electron spin Hamiltonian, incorporating Zeeman interactions and the dominant hyperfine couplings for both the FAD•− and Trp•+ radicals. Open System Evolution: Integration of the Lindblad master equation to track the time evolution of the singlet and triplet state populations under biological decoherence channels (spin relaxation, environmental dephasing, and radical recombination). Quantum Fisher Information (QFI): Calculation of the symmetric logarithmic derivative and QFI to determine the fundamental angular sensitivity limits of the radical pair compass. Information Theory: Computation of the Shannon mutual information and channel capacity between the Earth's magnetic field orientation and the molecular signaling states.
A ligand can adopt multiple conformations within the same binding site, and these conformations can interact differently with the surrounding protein. Multiconformer modeling must therefore determine which candidate conformations to retain and what occupancies to assign to them. An established automated approach uses mixed-integer quadratic programming (MIQP) with hard occupancy and conformer-count limits for this selection. Here, we reformulated conformer selection as a quadratic unconstrained binary optimization (QUBO) problem with finite penalties and discrete occupancies. We solved the QUBO using a quantum-classical hybrid solver. We compared this workflow with the MIQP-based qFit-ligand workflow across 83 paired X-ray ligand entries using the same experimental inputs and the same qFit-ligand processing protocol. For each entry, both final models were evaluated against the same composite-omit map generated from the common input structure before the two workflows diverged. Quantum-qFit produced higher ligand-region real-space correlation coefficients (RSCC) in 62 entries 74.7% and lower occupancy-weighted torsional strain in 66 entries 79.5%. Similar directional trends remained in the 28 entries with matched final conformer counts. The molecular Electron Density for Individual Atoms score (EDIAm) decreased in most entries. This result shows that higher regional map agreement did not necessarily correspond to stronger atom-level density support. Overall, the results show that the conformer-selection formulation can change which ligand states are represented from the same experimental data and provide a QUBO framework for studying multiconformer ligand selection.