Calculation of macroscopic first-, second-, and third-order optical susceptibilities for the urea crystal

Abstract

Macroscopic first-, second-, and third-order susceptibilities of the urea crystal are calculated using static and frequency-dependent ab initio molecular (hyper)polarizabilities at the self-consistent field (SCF) and second-order-Mo/ller–Plesset (MP2) levels. Environmental effects are taken into account using a rigorous local-field theory and are compared with the anisotropic Lorentz field factor approximation. The electric field arising from the permanent dipoles of the surrounding molecules is explicitly taken into account using a self-consistent approach. The dipole moment and the first hyperpolarizability are particularly strongly dependent on this field, but the crystal structure leads to a fortuitous cancellation of the field effect on the second-order susceptibility. The experimental linear susceptibility is accurately reproduced, while differences remain for the quadratic susceptibility. Dispersion curves for the first-order susceptibility, and results for quadratic electrooptic effect (QEO), electric-field-induced second-harmonic generation (EFISH), and third-harmonic generation (THG) experiments are predicted. The (hyper)polarizabilities of a linear dimer of urea molecules are calculated and used to estimate the effect of hydrogen bonding on the susceptibilities, which proves to be small. Semiempirically calculated (hyper)polarizabilities methods yield unreliable results for the susceptibilities compared with those from the ab initio method. This deficiency can be overcome by recourse to additional experimental data.

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

“…Evidence for this comes from the fact that the MP2/ DZP+ results ͑rather than the HF/ DZP+ results͒ most closely resemble the x-ray fitted results. The x-ray fitted results are also in remarkable agreement with MP2 calculations by Reis et al on an isolated molecule subject to a static applied electric field mimicking the crystal field, 14 ͑␣ xx , ␣ yy , ␣ zz ͒ = ͑4.54, 7.04, 7.01͒ ϫ 10 −40 C m 2 V −1 . The ab initio results obtained in that study also indicate that the effect of electron correlation on the dipole polarizability is substantial for urea, in line with our present findings and comments made by Birkedal et al 24 in their charge density study, and this can be partly attributed to the presence of a highly polarized C v O bond.…”

“…44 A rigorous treatment of the local field which takes into account field-induced dipolar interactions has been outlined by Dunmur, 44 described in considerable detail by Hurst and Munn, 45 and summarized in the recent papers by Reis et al [13][14][15][16] In this rigorous local field theory ͑RLFT͒ the electric susceptibility tensor for a molecular crystal with Z molecules per unit cell ͑volume V͒ can be expressed in the form…”

“…Various "local field factors" are used to approximate the effects of the surroundings of each molecule, and it is the more sophisticated treatment of these factors, usually by considering local dipole interactions between the molecules, that distinguishes interaction schemes from the oriented gas model. An interaction model forms the basis of recent studies by Reis et al on crystals of benzene, 13 urea, 14 naphthalene, anthracene, and m-nitroaniline ͑mNA͒, 15 several ice polymorphs, 16 ␣-sexithiophene, 17 and the disordered crystal 1,5-dichloro-2,3-dinitrobenzene. 18 In that approach, ab initio calculations on isolated molecules yield molecular ͑hyper͒polarizabilities which are in turn directly related to the macroscopic susceptibilities ͑ ͑1͒ , ͑2͒ , etc.͒ through a rigorous treatment of the local field as the sum of contributions from external fields as well as the field arising from surrounding molecules in the crystal.…”

DFT calculations of static dipole polarizabilities and hyperpolarizabilities for the boron clusters Bn (n=3-8,10)

“…The average axial bond length is with 1.71 Å considerably larger than that of the equatorial atoms with 1.60 Å [18]. The values of the first hyperpolarizability β calculated for the noncentrosymmetric clusters are comparably large: They are of the same order of magnitude as those of urea [22].…”

Theoretical modeling of the linear and nonlinear optical properties of organic crystals within the rigorous local field theory (RLFT)

“…This effect has been modeled either by the use of uniform electric dipole field, i.e. in [6][7][8] or more recently by non-uniform point charges field, i.e. in [9].…”