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It is shown that the low frequency collective modes of quasilinear polymers can contribute to the vibrational polarizability and hyperpolarizability despite the fact that their infrared or Raman intensity vanishes in the infinite chain limit. The most significant of these modes are characterized and the dynamic vibrational nonlinear optical (NLO) properties are obtained, at the doubly harmonic Hartree–Fock level of approximation, from static field calculations on polyacetylene as well as polyyne, polysilane, and polyethylene. Comparison is made to a pair of unit cell modes, one Raman-active and the other infrared-active, that are expected to generate a large vibrational hyperpolarizability. The combined effect of the collective and unit cell vibrations is potentially of considerable importance for the enhancement of NLO properties. Further studies are suggested to exploit this potential. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio restricted Hartree–Fock 6-31G calculations are reported for the static longitudinal hyperpolarizability of the linear polyenes C4H6 through C44H46. Using a new extrapolation technique the infinite chain value in polyacetylene is determined with an accuracy similar to that achieved for small molecules. This is the first in a series of articles leading to a comprehensive ab initio treatment for the nonlinear optical properties of conjugated polymers. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The local space approximation (LSA) method for embedding a finite cluster into its surroundings is implemented at the ab initio restricted Hartree–Fock level of theory. Our procedures for handling the initial combination of fragments in a way that takes full account of overlaps, and for obtaining the transformed two-electron integrals used in the local space self-consistent field (SCF) treatment, are presented in detail. Preliminary tests on small systems give results similar to those obtained in previous semiempirical calculations; in every case, the smallest physically meaningful local space yields an energy within about 1 kcal/mol of the ‘‘exact’’ full space value. [ABSTRACT FROM AUTHOR]

Dynamic vibrational first and second hyperpolarizabilities, applicable to a number of nonlinear optical processes, have been calculated for H2O, CO2, and NH3 at a typical laser frequency. As a percentage of their electronic counterparts, they range from being very small to being on the order of 20–30 %. The static values have also been calculated and are much larger. The calculations were based on perturbation theory and required knowledge of (a) first and second derivatives of the electrical properties (dipole moment, polarizability, and first hyperpolarizability functions) with respect to the normal coordinates, and (b) the harmonic and first anharmonic force constants. These were found at both the SCF and MP2 levels of approximation. In most cases electron correlation has a marked effect on the vibrational hyperpolarizabilities. Anharmonicity contributions are relatively small except for the static case and for the dc-Kerr effect. [ABSTRACT FROM AUTHOR]

Linear and nonlinear static electronic polarizabilities of the σ-conjugated polymer trans-polysilane (t-PSi) are obtained at the Hartree–Fock level by extrapolation of ab initio calculations on the oligomers H3Si–(SiH2)N–SiH3 (N odd). For the longitudinal components, in particular, a 6–31 G basis is shown to yield accurate values if the chains are long enough. No computational advantage is found in using effective core potentials. Chain lengths through N=13 are sufficient to give limiting infinite chain polarizabilities with relatively small uncertainty. The calculated longitudinal linear polarizability per SiH unit is about the same as the corresponding property in the prototype π-conjugated polymers polyacetylene (PA) and polydiacetylene (PDA), but the longitudinal second hyperpolarizability is significantly less in the σ-conjugated polymer. In t-PSi the conjugation length is about half that in either PA or PDA. Frequency-dependent vibrational distortion contributions are estimated in the doubly harmonic approximation and turn out to be important in trisilane and pentasilane even at optical frequencies. An analysis is developed which sheds light on the expected behavior for increasing chain lengths. Reasonable agreement with experiment is obtained for both the electronic and vibrational longitudinal second hyperpolarizabilities. [ABSTRACT FROM AUTHOR]

The spin distribution of the neutral soliton in trans-polyacetylene is controversial, as evidenced by the markedly different interpretations given by the various active experimental groups to their ENDOR measurements of proton hyperfine coupling (hfc) constants. To help resolve the discrepancies, ab initio electronic structure calculations were carried out for proton and carbon-13 hfc on the series of model polyene radicals C5H7,C9H11,...,C21H23 and the most important characteristics of the spin distribution were extrapolated to the long chain limit. Here σ spin polarization was treated through single excitation configuration interaction, both with respect to Hartree–Fock and to multiconfiguration reference spaces that include the most significant effects of π electron correlation. Particular attention was paid to basis set requirements necessary for proper treatment of hfc. It turns out that the ab initio hfc does not agree with that obtained by any of the major experimental groups. In particular, the calculated width of the bell-shaped distribution for the central and other even proton sites is much narrower than all experimental reports, and the anisotropic coupling calculated for the central proton along the fibril axis is much larger. Several assumptions commonly made in analyzing the ENDOR spectra are shown not to be quantitatively justifiable, but their improvement is unlikely to resolve the large discrepancies we have found between theory and current experimental interpretations. Thus, it appears likely that the experimental spectra are significantly influenced by environmental effects and cannot be adequately modeled by an isolated long chain polyene radical as heretofore assumed. [ABSTRACT FROM AUTHOR]

Perturbation formulas are derived for calculating the vibrational dynamic polarizability (αv) and hyperpolarizabilities (βv and γv ) of polyatomic molecules. These formulas, based on an initial harmonic oscillator approximation, include corrections for mechanical anharmonicity (cubic) terms in the vibrational potential and electrical anharmonicity (quadratic) terms in the dependence of the electrical field polarization potential on nuclear coordinates. Results are presented for FH and CO2. In the former case, comparison is made to ‘‘exact’’ numerical values. Fully documented computer programs for the perturbation treatment are available. [ABSTRACT FROM AUTHOR]

The SCF density matrix equations of the local space approximation are differentiated analytically. This is done through a polymatrix treatment which provides immediate generalization from ordinary Hartree–Fock to open-shell and MCSCF wave functions. Expressions are obtained in a form that is easily extended to arbitrary order. [ABSTRACT FROM AUTHOR]

The local space approximation (LSA) method is applied to a model Hamiltonian describing the chemisorption of a hydrogen atom onto a transition metal (0,0,1) surface. In the LSA method a finite cluster defined by a local (orbital) space is properly embedded into the complete system. Without embedding the calculated binding energy, ΔEb oscillates strongly as the size of the cluster increases, and even when 200–250 metal atoms are included the convergence is unsatisfactory. On the other hand, the LSA treatment yields rapid monotonic convergence. Using a systematic buildup of the local space the error in ΔEb is 4.4 kcal/mole out of 80.5 for a 10 metal atom cluster; that error decreases steadily to 1.8 kcal/mole for a cluster containing 30 metal atoms. Extrapolation reduces the error for all clusters between 18 and 71 metal atoms by about 0.6 kcal/mole. [ABSTRACT FROM AUTHOR]

The local space approximation (LSA) is applied to neutral and positive solitons, S0 and S+, in polyacetylene. In the treatment presented here a solitonic fragment is embedded between two finite polyene side chains. On the basis of INDO model calculations we conclude that electronic structure properties of the infinite system can be represented with chemical accuracy by a C9 solitonic fragment bonded to C4 or C6 side chains in S0 and C10 side chains in S+. The local space for each bond is defined by a C6H4 interaction complex augmented, for S+, with the π orbital of one additional carbon on the solitonic fragment and three additional carbons on the side chain. These results establish the feasibility of using the LSA method to carry out accurate ab initio Hartree–Fock and CI calculations based on a finite chain model for the soliton. [ABSTRACT FROM AUTHOR]

A local space approximation developed originally for the Hartree–Fock model of electronic structure is extended to configuration interaction and coupled cluster wave functions. In doing so many features employed by Saebo and Pulay in their local CI method are used to advantage. The computational effort is reduced to operations primarily on the local space (≡interaction complex) thereby making it feasible to study localized electronic structure changes in large systems. Charge transfer and polarization effects due to embedding the interaction complex in the surrounding medium are taken into account. [ABSTRACT FROM AUTHOR]

Studies the linear and nonlinear polarizabilities of polydiacetylene and polybutatriene chains. Extrapolation of the polarizabilities to the infinite polymer limit; Electronic and vibrational components obtained; Sensitivity of the components to the difference in bond length alternation in different ways.

Nonlinear optical properties of the linear polyenes C4H6 through C30H32 are calculated at frequencies below the first resonance using ab initio time-dependent Hartree–Fock theory. The results are used to determine accurate values for the infinite polymer, polyacetylene, by means of a new extrapolation method. Dispersion coefficients describing the low frequency behavior are obtained for the largest oligomer. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The evolution in the static longitudinal polarizability and second hyperpolarizability of the C4H2 through C54H2 oligomers of polyyne is studied at the Hartree–Fock level of theory. We find that the calculated values are highly sensitive to the adopted geometry. Using improved geometries, new extrapolation procedures and careful finite field determinations we extend earlier work by others so as to provide reliable estimates for the above properties in the infinite polymer limit. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electronic and vibrational contributions to the linear and nonlinear optical properties of the HF dimer have been calculated for the first time. The vibrational components are very significant with mechanical anharmonicity effects, as determined by perturbation-theoretic formulas, playing a major role. We identify the important anharmonic potential constants and analyze the validity, as well as the possible extension, of our theoretical treatment. Parallel computations, for purposes of comparison, have also been performed for the monomer. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]