Ab Initio Polarizabilities Research Articles

Evaluating non-linear optical behavior in ionic liquids: Combining ab-initio polarizability with additivity rule

In the field of non-linear Optics, the additivity rule, which both experimental and theoretical researchers utilize, provides a simple and reliable method for calculating polarizability. However, this model neglects crucial factors, such as the solvent, the interaction distance, and external optical fields, leading to inaccurate results. In this study, the role of these parameters on the non-linear optical behavior of the ionic liquid BMIM+⋯BF4−, a system extensively studied for its intriguing non-linear optical properties, is investigated. To do that, first-principles calculations based on the density functional theory are developed; the Lennard-Jones potential is used as the reference model. The results of this study reveal the significant influence of the solvent on the second- and third-order non-linear optical responses, which are strongly affected by ion interactions, underscoring the limitations of the additivity rule in terms of molecular polarizability. The EFISHG values for each solvent are inversely proportional to the dielectric constant; for all considered solvents, it observed a clear dependence of polarizability values on the external field, ranging from 1.675 to 1.638× 10−23 esu. It was also found high sensitivity of the static polarizability to changes in the interaction distance and the solvent choice, ranging from 1.57 to 2.05× 10−30 esu, while considering the additivity rule, this parameter changed to 1.59× 10−30 esu. This information can serve as a reference for both theoretical and experimental researchers.

Static polarizability surfaces of the van der Waals complex CH4–N2

The static polarizability surfaces of the van der Waals complex CH(4)-N(2) have been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules. The calculations have been carried out at the CCSD(T) and MP2 levels of the theory using the aug-cc-pVTZ basis set with the BSSE correction and within the framework of the classical long-range multipolar induction and dispersion interactions. It was shown that the results of analytical polarizability calculations for the CH(4)-N(2) complex are in a good agreement with the ab initio polarizabilities in the outer part of the van der Waals well on the complex potential surface. Ab initio calculations of the polarizability tensor invariants for the complex being in the most stable configurations were carried out. The change in the polarizability of CH(4)-N(2) due to the deformation of the CH(4) and N(2) monomers at the formation of the complex was estimated. In the framework of the analytical approach the polarizability functions alpha(ii)(R) of the free oriented interacting molecules CH(4) and N(2) were calculated.

Molecular Dynamics Simulations of a DMPC Bilayer Using Nonadditive Interaction Models

We present a polarizable force field based on the charge-equilibration formalism for molecular dynamics simulations of phospholipid bilayers. We discuss refinement of headgroup dihedral potential parameters to reproduce ab initio conformational energies of dimethylphosphate calculated at the MP2/cc-pVTZ level of theory. We also address the refinement of electrostatic and Lennard-Jones (van der Waals) parameters to reproduce ab initio polarizabilities and water interaction energies of dimethylphosphate and tetramethylammonium. We present results of molecular dynamics simulations of a solvated dimyristoylphosphatidylcholine bilayer using this polarizable force field as well as the nonpolarizable, fixed-charge CHARMM27 and CHARMM27r force fields for comparison. Calculated atomic and electron-density profiles, deuterium order parameters, and headgroup orientations are found to be consistent with previous simulations and with experiment. Polarizable interaction models for solvent and lipid exhibit greater water penetration into the lipid interior; this is due to the variation of water molecular dipole moment from a bulk value of 2.6 Debye to a value of 1.9 Debye in the membrane interior. The reduction in the electrostatic component of the desolvation free-energy penalty allows for greater water density. The surface dipole potential predicted by the polarizable model is 0.95 V compared to the value of 0.8 V based on nonpolarizable force-field calculations. Effects of inclusion of explicit polarization are discussed in relation to water dipole moment and varying charge distributions. Dielectric permittivity profiles for polarizable and nonpolarizable interactions exhibit subtle differences arising from the nature of the individual component parameterizations; for the polarizable force field, we obtain a bulk dielectric permittivity of 79, whereas the nonpolarizable force field plateaus at 97 (the value for pure TIP3P water). In the membrane interior, both models predict unit permittivities, with the polarizable models contributing from one to two more units due to the optical dielectric (high-frequency dipole fluctuations). This contribution is a step toward the continuing development of a CHARMM (Chemistry at Harvard Molecular Mechanics) polarizable force field for simulations of biomacromolecular systems.

Density Functional Studies of the Dipole Polarizabilities of Substituted Stilbene, Azoarene and Related Push-Pull Molecules

We report high quality B3LYP Ab Initio studies of the electric dipole polarizability of three related series of molecules: para-XC6H4Y, XC6H4CH=CHC6H4Y and XC6H4N=NC6H4Y, where X and Y represent H together with the six various activating through deactivating groups NH2, OH, OCH3, CHO, CN and NO2. Molecules for which X is activating and Y deactivating all show an enhancement to the mean polarizability compared to the unsubstituted molecule, in accord with the order given above. A number of representative Ab Initio calculations at different levels of theory are discussed for azoarene; all subsequent Ab Initio polarizability calculations were done at the B3LYP/6-311G(2d,1p)//B3LYP/6-311++G(2d,1p) level of theory. We also consider semi-empirical polarizability and molecular volume calculations at the AM1 level of theory together with QSAR-quality empirical polarizability calculations using Miller’s scheme. Least-squares correlations between the various sets of results show that these less costly procedures are reliable predictors of for the first series of molecules, but less reliable for the larger molecules.

Many-body force field models based solely on pairwise Coulomb screening do not simultaneously reproduce correct gas-phase and condensed-phase polarizability limits.

It is demonstrated that many-body force field models based solely on pairwise Coulomb screening cannot simultaneously reproduce both gas-phase and condensed-phase polarizability limits. Several many-body force field model forms are tested and compared with basis set-corrected ab initio results for a series of bifurcated water chains. Models are parameterized to reproduce the ab initio polarizability of an isolated water molecule, and pairwise damping functions are set to reproduce the polarizability of a water dimer as a function of dimer separation. When these models are applied to extended water chains, the polarization is over-predicted, and this over-polarization increased as a function of the overlap of molecular orbitals as the chains are compressed. This suggests that polarizable models based solely on pairwise Coulomb screening have some limitations, and that coupling with non-classical many-body effects, in particular exchange terms, may be important.

The Raman spectra and cross-sections of the ν2 band of H 2O, D 2O, and HDO

We report the experimental Raman spectra of the ν 2 band of H 2O, D 2O, and HDO in the vapor phase at room temperature. A complete interpretation of the Raman intensities is carried out employing the variational rovibrational wavefunctions obtained from a Hamiltonian in Radau coordinates and an ab initio polarizability surface at 514.5 nm. We show the importance of the rotation–vibration coupling to obtain the correct line intensities. Several tables with the assignments of the individual rotational–vibrational transitions and their Raman scattering strengths are reported. From these tables, the ν 2 Raman spectra can be simulated up to 2000 K for H 2O, and up to 300 K for D 2O and HDO.

Ab Initio Atomic Polarizability Tensors for Organic Molecules

A recently developed method for deriving anisotropic atomic dipole polarizability tensors has been employed to derive these terms for each of the atoms in seven families of organic compounds: alcohols, alkanes, amides, amines, carboxylic acids, sulfur compounds, and benzene rings. The procedure uses fits to ab initio quantum mechanically computed molecular polarizability data to determine effective atomic polarizability tensor parameters. A new representation is described which uses bond increments, a property of the bonds rather than simply the atomic types, for polarizability components both parallel and perpendicular to the bonds. It is shown to give excellent results as gauged by its reproduction of the exact ab initio polarizabilities. We examine the accuracy of the ab initio isotropic polarizabilities compared to experiment and present a simple scaling procedure for the former. The isotropic approximations to the full anisotropic atomic polarizabilities are compared for differing atom types and bonding environments. The resulting molecular isotropic polarizabilities are also compared where possible with experiment. The present model consistently predicts both experimental isotropic molecular polarizabilities and empirically derived atomic parametrizations precisely within a few percent.

Ab initioprediction of the electronic and optical excitations in polythiophene: Isolated chains versus bulk polymer

We calculate the electronic and optical excitations of polythiophene using the GW (G stands for one-electron Green function, W for the screened Coulomb interaction) approximation for the electronic self-energy, and include excitonic effects by solving the electron-hole Bethe-Salpeter equation. Two different situations are studied: excitations on isolated chains and excitations on chains in crystalline polythiophene. The dielectric tensor for the crystalline situation is obtained by modeling the polymer chains as polarizable line objects, with a long-wavelength polarizability tensor obtained from the ab initio polarizability function of the isolated chain. With this model dielectric tensor we construct a screened interaction for the crystalline case, including both intra- and interchain screening. In the crystalline situation both the quasiparticle band gap and the exciton binding energies are drastically reduced in comparison with the isolated chain. However, the optical gap is hardly affected. We expect this result to be relevant for conjugated polymers in general.

Molecular and Atomic Polarizabilities: Thole's Model Revisited

Thole's modified dipole interaction model for constructing molecular polarizabilities from effective, isotropic atomic polarizabilities is reviewed and extended. We report effective atomic polarizabilities for H, C, N, O, S, and the halogen atoms, independent of their chemical environment. They are obtained by fitting the model both to experimental and calculated molecular polarizabilities, the latter to enable one to model ab initio polarizabilities for various basis sets.

Realistic representation of the induced electric dipole moment of a polarizable ligand: The missing factor in the Rittner polarization model

We present a simple modification of the Rittner polarization model1 to better represent the dipole moment of a negative ion induced by the electric field of its counterion. At high fields the traditional second-order perturbation theory treatment of ion polarization is not appropriate. The internuclear separation dependence of the induced dipole moment is better described by explicitly modeling the mixing of free ion basis states. The dipole moment function of the ion is evaluated using the ab initio value of the free ion polarizability and its experimentally determined electron affinity. We present a simple and quantitative resolution of the mysterious difference between ab initio polarizabilities and Rittner model-derived values, which previously had been optimized separately for every different counterion.

An ab Initio and Semiempirical Study of the First- and Third-Order Polarizabilities in Benzene and Thiophene Derivatives: Electron Correlation Effects

The static first- and third-order polarizabilities of several benzene and thiophene derivatives are evaluated at the ab initio level via an efficacious general finite field approach. The impact of electron correlation is explored by calculating the molecular polarizabilities at the Moller−Plesset second-order perturbation (MP2) theory level using an extended basis set. Further, we examine the influence of molecular architecture on the nonlinear optical response, in particular the switch from an arylethenyl type of structure to a quinoid structure. Finally, we apply the correction vector method combined with the intermediate neglect of differential overlap−single and double excitation configuration interaction (INDO-SDCI) technique to evaluate the optical nonlinearities at the semiempirical level. The reliability of this procedure is established by comparison with the corresponding high-level ab initio polarizability values. The results clearly show the sensitive dependence of the nonlinear optical propert...

Ab initio polarizabilities of polyenic chains with conformational defects

We present an ab initio Hartree-Fock study of the electronic polarizabilities of the C 2 n+1 H 2 n+3 + (2 < n < 11) and C 2 n H 2 n+2 ++ (2 < n < 15) oligomers of polyacetylene. After a complete geometry optimization implemented through the GAUSSIAn 92 program, the longitudinal components of the linear polarizabilities were analytically determined and the second hyperpolarizabilities calculated through a finite field procedure. While the first hyperpolarizabilities of the bipolaron-like structures vanish (since inversion symmetry is preserved), the dominant component β xxy of the soliton chains was obtained analytically. We confirm that the polarizabilities of these polyenic oligomers are quite sensitive to the nature of the conformational defect present.

Determination of ab initio polarizabilities of polymers: Application to polyethylene and polysilane

AbstractAn ab initio method for calculating the longitudinal linear polarizability of polymeric chains is described. This method is equivalent to an uncoupled Hartree–Fock scheme. It is applied to polyethylene and polysilane in minimal STO‐3G and extended 4‐31G basis sets. The study describes important techniques for solving the difficulties met in actual calculations: band reordering of the band structures, calculation of analytical derivatives of the energy bands ϵn(k) and LCAO coefficients cnp(k), and errors caused by the improper lattice sum truncations of the Hartree–Fock matrix.

The three-body contribution to the polarizability of a trimer of inert gas atoms using a dipole—induced-dipole model

Expressions are derived, using the classical interacting-dipole model, for the three-body contribution to the polarizability of a triplet of inert gas atoms in arbitrary configuration. Comparison is made to ab initio polarizability calculations by Pérez, Clarke and Hinchliffe for a He 3 cluster in linear and isosceles-triangular configurations. Significant discrepancies are found, even at large separations between the atoms.