Applicability of the unit-cell polaron method to anharmonic, disordered, and liquid systems

Ascertain the validity and applicability of the unit-cell polaron method based on quadratic expansions of lattice displacements and neglect of higher-order (anharmonic) contributions in systems that are strongly anharmonic, disordered, or liquid (e.g., the hydrated electron in water), and determine if modifications are required to handle such cases.

Background

The unit-cell method of Sio et al. constructs polaron energetics using electron–phonon couplings from a neutral undistorted unit cell and expands the energy cost of lattice distortions to second order (harmonic approximation), neglecting higher-order terms. The authors point out that anharmonic effects can be crucial in disordered and liquid environments and may affect polaron states, potentially invalidating the harmonic truncation.

Because pSIC does not require such an approximation and can include anharmonic contributions via total energy differences, the authors stress that it is unknown whether the unit-cell method remains reliable in these settings, highlighting the need to assess and possibly extend the formalism.