Photoinduced charge separation underlies many processes, including photosynthesis, singlet fission, and photocatalysis. The dynamics of such processes depend crucially on the crossings and couplings of valence and charge-transfer (CT) states; however, resolving the latter experimentally is challenging given their dark nature, motivating theoretical approaches that are both fully atomistic and predictive, while computationally affordable for condensed-phase systems. Multiscale modelling is a natural choice, wherein the chromophores are treated quantum mechanically (QM) with the remainder of the system using molecular mechanics (MM) forcefields; however, CT states in particular require a description of electronic polarization of the environment i.e. with polarizable embedding (PE). While current PE methods accurately model vertical excitations, little is known about their global description of potential energy surfaces. We show that many PE methods exhibit serious artifacts near conical intersections (CI), which are vital to understanding photochemistry. The issue can be traced to the need to simultaneously describe separate environment polarizations in response to each electronic state. Building on an old idea from Thole, we solve this problem by developing an integral exact direct reaction field (IEDRF) method. All electronic states are eigenfunctions of the same Hamiltonian, while the polarization induced in the environment is unique to each state, thus CIs are described correctly. The embedding operator contains 1 and 2-electron terms that directly screen electron-electron repulsions and thus account for the stabilization of CT states. Formulating IEDRF in terms of effective Coulomb and exchange matrices allows efficient plug-in compatibility with e.g. HF, CIS, and CASSCF electronic structure methods and their analytic gradients. After demonstrating IEDRF on a prototypical CI of ethylene, we apply it to the primary CT states of the photosynthetic purple bacterial reaction center (PBRC). Despite having two branches that are almost symmetric, PBRC displays photoinduced charge separation along a single branch, the origin of which is still under debate. We find IEDRF predicts asymmetry in the CT state energies of the PBRC branches that correlate with differences in local fields experienced by the chromophores in quantitative agreement with Boxer’s recent measurements.

Modelling conical intersections in the condensed phase with multiscale polarizable embedding

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