High-pressure-driven covalent network reconstruction in π-stacked CL- 20/NAQ cocrystals: a first-principles study.

CONTEXT: This study systematically investigated the hydrostatic pressure response (0-100 GPa) of CL-20/NAQ cocrystals through an integrated analysis of crystallographic, molecular, electronic, optical, mechanical, and Hirshfeld surface properties. The computational results reveal discontinuous structural reorganizations within the 70-100-GPa range, characterized by covalent bond rearrangement involving concurrent cleavage of the C14-H14 bond and the formation of new intermolecular linkages (C9-C15, C10-C14, C8-C14, O4-H14, C8-O13, and O2-H10). These phase transformations manifested as discontinuous variations in the lattice parameters, covalent bond lengths, density of states profiles, and specific optical properties. The computed optical properties include the optical absorption coefficient, dielectric function (real and imaginary parts), electrical conductivity, reflectivity, energy loss function, and refractive index, which collectively exhibit pressure-induced modulation patterns consistent with structural and electronic reorganizations. The multiscale characterization approach included crystallographic configuration evolution, molecular architecture modifications, electronic structure reorganization, optical response modulation, mechanical property transformations, and Hirshfeld surface topology analysis. All the observed transitions were validated through self-consistent convergence of evidence across complementary methodologies, demonstrating pressure-driven covalent reconfiguration in energetic cocrystal systems.

METHOD: This investigation employed density functional theory (DFT) simulations using the CASTEP module within Materials Studio 2023. The 1:1 CL-20/NAQ cocrystal system underwent full geometric optimization across 0-100 GPa hydrostatic pressure conditions via the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. Initial structural relaxation at ambient pressure was performed without constraints using the local density approximation with HSE06. All calculations were implemented with a plane-wave basis set with a 489-eV kinetic energy cutoff, ensuring self-consistent convergence of unit cell total energies below 1 × 10-6 eV/atom. The Brillouin zone was sampled exclusively at the Γ-point (2 × 2 × 2 k-mesh), with hydrostatic pressure explicitly applied through the stress tensor prior to each electronic minimization cycle. This methodology maintained consistent convergence criteria of 0.01 eV/Å for the atomic forces, 0.05 GPa for the stress components, and 1 × 10-5 eV for the total energy throughout the pressure series. To further validate the accuracy of the frequency-dependent optical properties, we performed a convergence test using a stricter threshold of 0.005 eV/Å, which yielded negligible differences (less than 2% variation in peak intensities) compared with the 0.01 eV/Å results, confirming that our selected criteria are sufficient.