Surfaces, Shapes, and Bulk Properties of Crystals

We study the interplay between surface and bulk properties of macroscopic materials. It is demonstrated that the so-called polar surfaces may be stabilized through a charge redistribution between the complete set of surfaces that depends upon the overall shape of the sample and the nature of the material. This charge redistribution, in turn, is governed by certain constraints that we call generalized Tasker conditions. The same surface, but for samples of different shapes, may have different surface charges. Besides its stabilizing effect, the charge redistribution is also shown to particularly affect the dipole moment per repeat unit, a bulk property. For the latter, it is established that essentially any physically meaningful value is possible (depending upon the shape and material), in contrast to the often made assumption that different samples of the same material will have values that differ by, at most, a lattice vector. Finally, some recent experimental and theoretical results for polar surfaces are discussed in terms of the analysis presented here

Electronic and Vibrational Nonlinear Optical Properties of Five Representative Electrides

The electrides have a very special electronic structure with diffuse excess electrons not localized on any specific atom. Such systems are known to have huge electronic nonlinear optical (NLO) properties. Here, we determine and analyze the vibrational, as compared to the electronic, NLO properties for a representative set of electrides: Li@Calix, Na@Calix, Li@B₁₀H₁₄, Li₂^•+TCNQ^•–, and Na₂^•+TCNQ^•–. The static and dynamic vibrational (hyper)­polarizabilities are computed by the nuclear relaxation method (with field-induced coordinates and the infinite optical frequency approximation) at the UB3LYP level using a hybrid Pople basis set. In general, the static vibrational β_vec and γ_∥ exceed the corresponding static electronic property values by up to an order of magnitude. The same comparison for dynamic vibrational hyperpolarizabilities shows a smaller ratio. For the intensity-dependent refractive index (IDRI) and dc-Kerr processes, the ratio is on the order of unity or somewhat larger; it is less for the dc-Pockels and the electric field induced second harmonic (EFISH) effects (as well as the static α̅) but still important. The role of anharmonicity, motion of the alkali atoms, and substitution of Na for Li is discussed along with specific aspects of the charge distribution associated with the excess electron

A Full Dimensionality Approach to Evaluate the Nonlinear Optical Properties of Molecules with Large Amplitude Anharmonic Tunneling Motions

Previously, a reduced dimensionality approach was used to determine the vibrational contribution to nonlinear optical properties for molecules with large amplitude anharmonic modes that takes into account tunneling between potential wells (Luis, J. M.; Reis, H.; Papadopoulos, M. G.; Kirtman, B. <i>J. Chem. Phys.</i> <b>2009</b>, <i>131</i>, 034116). Here, the treatment is extended, again using ammonia as an example, to include the remaining modes at several approximate levels. It is shown that this extension is essential to obtaining the correct results. Our new approach fully accounts for tunneling and avoids possible convergence problems associated with the normal coordinate expansion of the potential energy surface in a single-well treatment. For accurate numerical values, a good treatment of electron correlation is required along with a flexible basis set including diffuse functions