QTAIM-Based Scheme for Describing the Linear and Nonlinear Optical Susceptibilities of Molecular Crystals Composed of Molecules with Complex Shapes

The Quantum Theory of Atoms in Molecules (QTAIM) to distribute the molecular polarizability tensors over submolecular sites is employed to improve the prediction of local field tensors of molecular crystals and therefore of their linear and second-order nonlinear optical susceptibilities. This extension of the two-step multiscale procedure is intended to better describe crystals built of molecules having complex shapes. When combined with a simple charge embedding approach to account for the crystal field effects on the molecular (hyper)polarizabilities, this QTAIM local field theory (Q-LFT) approach is efficient to predict the _χ ⁽¹⁾ and _χ ⁽²⁾ tensor components. Moreover, it does not require substantial computational resources because the largest calculations are performed on the individual molecules. This is illustrated by considering derivatives of (S)-2-(α-methylbenzylamino)-5-nitropyridine (MBANP) as well as 2-methyl-4-nitroanline (MNA), which is a prototypical push-pull π-conjugated compound. In the latter case, the use of the Q-LFT only leads to minor differences in the _χ ⁽¹⁾ and _χ ⁽²⁾ tensor components with respect to the standard approach where the polarizability is equipartitioned. In the case of the MBANP derivatives, the Q-LFT scheme leads to systematic decrease of the linear and nonlinear optical responses. This generally improves the agreement between the calculated and experimental refractive indices and second-order nonlinear optical susceptibilities. Indeed, the standard partitioning scheme leads to an overestimation of the phenyl out-of-plane polarization component and therefore of the refractive indices. The _χ ⁽²⁾ variations among the MBANP derivatives have further been analyzed in terms of molecular geometry, crystal polarizing field, and crystal packing. (Chemical Equation Presented).