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Macroscopic first- and third-order susceptibilities of ice Ih, ice II, ice IX and ice VIII are calculated using static and frequency-dependent electronic and static vibrational molecular (hyper)polarizabilities at the MP2 level. The molecular properties are in good agreement with experiment and with high-level ab initio calculations. Intermolecular electrostatic and polarization effects due to induced dipoles are taken into account using a rigorous local-field theory. The electric field due to permanent dipoles is used to calculate effective in-crystal (hyper)polarizabilities. The polarizability depends only weakly on the permanent field, but the dipole moment and the hyperpolarizabilities are strongly affected. The calculated linear susceptibility is in good agreement with available experimental data for ice Ih, and the third-order susceptibility for a third harmonic generation experiment is in reasonable agreement with experimental values for liquid water. The molecular vibrational contributions have a small effect on the susceptibilities. The electric properties of a water tetramer are calculated and used to estimate the effect of non-dipolar interactions on the susceptibilities of ice Ih, which are found to be small.

The molecular structure and IR spectra of urea, H2NCONH2, in gas phase and in acetonitrile solution, as well as of the two complexes [MgU4Cl2] and [MgU6]Cl2 have been observed. The influence of environmental changes to geometry and spectra are shown. Various basis sets have been employed to safeguard the validity of the reported findings, using polarization functions for all calculations to get the correct pyramidal amide configuration. The erroneous low energy of the C2v symmetry group, after the addition of the ZPVE correction, is discussed. For the solvated urea molecule a reduction of the energy barrier, compared to the gas phase urea, between the two minimum configurations, C2 and Cs, and the planar geometry, is observed. The lowest energy minimum in acetonitrile is found to be the C2 symmetry group, while for the two complexes, the local symmetry of urea is Cs or C2 depending on the complex, or even on the coordination position of urea in the complex. The wagging motion of the amide group is also discussed in all the studied urea species. The computed geometries and most of the spectroscopic results are in good agreement with the available experimental data.

The electronic structure of the fully lithiated benzene molecule has been calculated at different levels of approximation with particular attention paid to its second hyperpolarizability. It has been estimated that lithiation of the benzene molecule may lead to the enormous increase of the second hyperpolarizability. For hexalithiobenzene the second hyperpolarizability term responsible for the third harmonic generation has been estimated to be of the order of 107e4ao4Eh−3. The reasons for the very large increase of the dipole polarizability, and in particular, of the second hyperpolarizability upon lithiation have been analysed in terms of the electronic structure, orbital energy levels, and the excitation spectrum of the hexalithiobenzene molecule. The main factor responsible for the particularly large values of the studied properties is the electronic valence shell structure of the lithium atom. Although the hexalithiobenzene molecule is used as a model for the study of the lithiation effect, several stable organometallic systems involving alkali metals are known. The present study suggests that among these systems one may find materials of extremly high nonlinear optical efficiency.

The structure-polarization relationship has been investigated in a series of polyenes. Various conjugation patterns have been studied and their effect on the␣polarizabilities and hyperpolarizabilities has been commented upon. Two classes of conjugated oligomers have been shown to have very large second hyperpolarizabilities. It is shown that the known potential of the conjugated chains to lead to large nonlinearities is enhanced by substituting one or more hydrogen atoms with lithium. This interesting and useful point is documented by calculating the hyperpolarizabilities of several selected organolithium derivatives presenting a variety of molecular structures. The largest computed nonlinearity is γ = 4 × 107 a.u., which is an exceptionally large value. The present computations have been performed using the semi-empirical approaches MNDO, PM3 and MNDO/d. This choice of well-tested semi-empirical models, in connection with the available literature values (theoretical and experimental), supports the validity of the reported findings.

Molecular dynamics (MD) simulation and subsequent analysis of the macroscopic polarization developed in response to “a posteriori” applied electric fields or of spontaneous fluctuations in the instantaneous polarization under zero applied field is used to assess the nonlinear optical properties of a polarizable liquid. Three strategies are proposed for the electrostatic analysis, all using as input static “gas phase” (hyper)polarizabilities, obtained from ab initio calculations. All three strategies are shown to accurately reproduce the experimentally measured refractive index and second nonlinear susceptibility of liquid benzene. The simulation also predicts the distribution of orientations and magnitudes of the local electric fields experienced by the molecules in the liquid, and the nonlinear contributions to the local fields. This approach gives an 8% higher estimate of the second nonlinear susceptibility of liquid benzene than the Lorentz local field factor approach, in better agreement with experimental values.

The polarizabilities and hyperpolarizabilities of C8H6, C8H6Li2 and C8H6Na2 have been computed using MP2 theory. Optimized structures have been employed. The effect of changes in the geometry on the properties is discussed. Distortions induced by lithium and sodium in the anionic moieties are documented. Evidence for the electronic instability of pentalenyl dianion is presented. It has been found that introduction of lithium and sodium into the molecular structure leads to derivatives with very large hyperpolarizabilities. It is thus proposed that organolithium and organosodium compounds are very likely to lead to materials with the required very large nonlinearities, which are necessary in many scientific and industrial applications. Molecular Physics

Static and dynamic polarizabilities and second hyperpolarizabilities are presented for ethylene, including electronic, pure vibrational and zero-point vibrational averaging (ZPVA) contributions. Static pure vibrational hyperpolarizabilities are of commensurate size with their electronic counterparts but show less dependence on basis set and (MP2) electron correlation. A Hartree–Fock estimate for the static ZPVA correction to γ is found to be 10% of the corresponding electronic contribution. The dynamic pure vibrational hyperpolarizability, despite being significant for the optical Kerr effect and intensity-dependent refractive index, reduces in magnitude for second and third harmonic generation, leaving ZPVA as the dominant vibrational effect.

The polarizability (α) and the second hyperpolarizability (γ) of tetrakis(phenylethynyl)ethene (TPEE) are compared and analyzed in connection with the properties (α, γ) of a series of selected/designed molecules having different conjugation patterns. Several lithiated derivatives of TPEE are designed and shown to have very enhanced second hyperpolarizabilities; for example, one of the lithiated TPEE has a 1.6×103 times larger second hyperpolarizability than that of benzene. The potential of the proposed derivatives for applications in photonics is noted. The polarizabilities and the hyperpolarizabilities of the considered molecules have been computed employing the PM3 method which has been proven to be adequate for the present comparative study. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 177–187, 1999

Dipole moments, linear and non-linear polarizabilities of [Mo(NO){HB(dmpz)3}Cl{OC6H4[CHCHC6H4N(CH3)2-4]-4} (dmpz = 3,5-dimethylpyrazolyl) (DMAMoS) and p,p′-dimethylaminonitrostilbene (DMANS) calculated at the SCF and second-order Moller–Plesset correlation (MP2) level with an effective core potential (ECP) basis set are reported. It is shown for DMANS that the ECPs are as accurate as calculations using all-electron basis sets of comparable quality. The effect of diffuse functions on the dominant components of the hyperpolarizabilities of DMANS is small. The dipole moment and linear polarisabilities of DMANS are in very good agreement with experimental values. The calculated hyperpolarisabilities are much smaller than the reported experimental values, but there are large uncertainties among the latter concerning calibration factors, concentration dependence and permanent local field effects, which render direct comparison difficult. Electronic correlation effects on the hyperpolarisabilities are large, especially for DMAMoS, for which the dominant component of the first hyperpolarisability increases by a factor of 5. Generally, the hyperpolarisabilities of DMANS are found to be larger than those of DMAMoS, in spite of the highly polarisable electronic charge-transfer character of the latter.

The polarizability and first and second hyperpolarizabilities of several 4,4'-disubstituted trans- stilbene (tSB) derivatives, which include the NO2 group as an acceptor (A) and several lithium containing donors (D) have been computed. Ab initio theory using a 6-31G basis set at the SCF and MP2 levels has been sued to optimize the structure of tSB and the disubstituted derivative which includes the NO2 (A) and the NLi2 (D). Their polarizabilities and hyperpolarizabilities have also been computed at the SCF level. MP2 theory has been used to assess the effect of correlation on the largest polarizability and second hyperpolarizability components of the above compounds. The preset results, together with our recent work show that the second hyperpolarizabilities of some lithiated derivatives depend quite a lot on the molecular geometries and lithiation leads to a big increase of the hyperpolarizabilities of the resulting derivatives. This finding suggests that at lest some of the lithiated derivatives are very likely to be useful for non-linear optical applications.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Starting from a set of high-level ab initio frequency-dependent molecular first- and third-order polarizabilities, the macroscopic first-order (linear) and third-order (cubic) susceptibilities of the benzene crystal are calculated. Environmental effects are taken into account using a rigorous local-field theory and are compared with the anisotropic Lorentz field factor approach. The experimentally determined first-order susceptibility of crystalline benzene is accurately reproduced. Dispersion curves for the first-order susceptibility and results for electric-field-induced second-harmonic generation and third-harmonic generation experiments are predicted. Comparison with similar calculations conducted in the course of molecular simulations of liquid benzene shows that the theoretical results for the two phases are of comparable accuracy. Overall, the results show that for the fairly compact nonpolar benzene molecules, environmental effects on the effective molecular response are small.