Effective molecular polarizabilities and crystal refractive indices estimated from x-ray diffraction data

Area of Science:

• Solid-state chemistry
• Computational chemistry
• Materials science

Background:

• Estimating molecular (hyper)polarizabilities from X-ray charge density refinements was previously unconvincing.
• Standard multipole refinement models lack sufficient information for accurate response property determination.

Purpose of the Study:

• To develop a reliable method for calculating molecular polarizabilities and related properties from X-ray diffraction data.
• To investigate the influence of intermolecular interactions and electron correlation on these properties in crystalline solids.

Main Methods:

• Utilizing a "constrained wave function" approach by fitting to X-ray structure factors.
• Deriving simplified sum-over-states expressions for dipole polarizability, including previously ignored two-electron terms.
• Calibrating results against coupled Hartree-Fock (ab initio) calculations to obtain in-crystal effective polarizability tensors.

Main Results:

• The constrained wave function approach accurately captures essential two- and three-electron terms for polarizability and hyperpolarizability calculations.
• Effective molecular polarizabilities for benzene, urea, and 2-methyl-4-nitroaniline demonstrably include intermolecular and electron correlation effects.
• A rigorous local field treatment using averaged Lorentz-factor tensors provides accurate predictions of linear bulk susceptibility and refractive indices, outperforming point-dipole approximations.

Conclusions:

• The constrained wave function method, combined with appropriate local field models, offers a robust route to determining accurate in-crystal polarizabilities and optical properties from X-ray diffraction.
• This approach is particularly valuable for materials with significant intermolecular interactions, such as urea, and for nonlinear optical applications.