First-principles determination of exciton dephasing via time-dependent adiabatic GW approach

Dephasing, the decay of off-diagonal density-matrix elements, sets the homogeneous optical linewidth and the visibility of coherent signals in time-resolved and 2D spectroscopy, and is driven primarily by exciton–phonon and exciton–exciton scattering. In the recently developed time-dependent adiabatic GW (TD-aGW) framework, the density matrix is propagated in real time, while dephasing is typically introduced phenomenologically by a factor e^−γt. We present an ab initio route to compute the exciton dephasing time at finite temperature arising from exciton–phonon scattering. The approach uses the special-displacement method to emulate finite-temperature lattice fluctuations by appropriately perturbing the atomic positions in a supercell. GW and GW-BSE calculations on perturbed supercells of monolayer MoS2 reproduce the observed finite-T redshift of the A-exciton, supporting the fidelity of the thermal sampling. We then perform TD-aGW on the same supercells and extract the exciton dephasing time. This procedure yields intrinsic homogeneous linewidths that increase with temperature and are consistent with the redshift trends captured by GW-BSE on displaced structures. The results provide a first-principles determination of exciton dephasing within TD-aGW without introducing an external dephasing parameter.