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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. J. Chem. Phys.2009, 131, 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.

The basic principles behind a theoretical method for treating infinite, periodic systems exposed to an external electrostatic field are outlined. The approach, based on the vector-potential description of the external field, leads to single-particle Hartree-Fock or Kohn-Sham equations that differ from the field-free counterparts in several aspects. In particular, solving them is only possible through a careful so-called smoothing procedure. In that case it is possible to derive a numerically stable and efficient approach. Results of model studies as well as of the first ab initiocalculations are reported in order to illustrate the approach. Finally, extensions and open issues are briefly discussed.

Static longitudinal polarizabilities (α) and second hyperpolarizabilities (γ) are determined for three quasi-one-dimensional polymeric systems using a new crystal orbital coupled-perturbed Hartree–Fock method. Calculations of α and γ for polydiacetylene and polybutatriene at two different geometries are analyzed in terms of the alternation of bond lengths along the carbon backbone. For the third polymer, polyacetylene, the treatment is applied to a pair of chains in the two different configurations that occur in a stretched fiber. The results are in marked contrast to those previously obtained for short and medium-sized oligomers. In order to relate the pair interaction effect between infinite chains to the bulk susceptibility of the natural fiber additional finite oligomer calculations are carried out. This allows us to discuss the role of finite chain length distribution, many-body interchain interactions, and electron correlation. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

The Crystalprogram for quantum-mechanical simulations of materials has been bridging the realm of molecular quantum chemistry to the realm of solid state physics for many years, since its first public version released back in 1988. This peculiarity stems from the use of atom-centered basis functions within a linear combination of atomic orbitals (LCAO) approach and from the corresponding efficiency in the evaluation of the exact Fock exchange series. In particular, this has led to the implementation of a rich variety of hybrid density functional approximations since 1998. Nowadays, it is acknowledged by a broad community of solid state chemists and physicists that the inclusion of a fraction of Fock exchange in the exchange-correlation potential of the density functional theory is key to a better description of many properties of materials (electronic, magnetic, mechanical, spintronic, lattice-dynamical, etc.). Here, the main developments made to the program in the last five years (i.e., since the previous release, Crystal17) are presented and some of their most noteworthy applications reviewed.

For purposes of interpretation and understanding, the valence bond orbital model of electronic structure provides a number of advantages over the more common molecular orbital model. This derives primarily from the unique description of valence bond orbitals in terms of highly localized bond pairs, lone pairs, etc. Optimization of the orbitals by a self‐consistent‐field procedure brings out features which were not evident from the semi‐empirical treatments of earlier workers. Calculations on CH4, NH3, H2O and H2S show that optimized valence bond orbitals are often bent even at the equilibrium nuclear configuration and generally do not follow the nuclei when bond angles are varied. Analysis of a large basis set calculation on H2O shows that the valence bond wavefunction can be fitted well with a minimal basis, making it possible to interpret more clearly the connection between hybridization and interorbital angles. It is shown how the valence bond method provides a rigorous basis for factorization even of difficult second order physical properties, such as polarizability, into separated bond and lone pair contributions. These contributions can be further decomposed into sums of atomic valence state values plus corrections due to bond polarization effects. Finally, the advantages of using the valence bond model as a starting point for the perturbation treatment of electron correlation are pointed out. Calculations on H2and LiH indicate that valence bond theory need only be carried through second order to obtain results comparable in accuracy to a third order treatment based on molecular orbitals.

We identify the energy contributions that govern the interorbital and internuclear angles in the classical covalent structure of H2O. The central atom valence state term plays a primary role in H2O and other AH2 molecules as well. Lone pair interactions of three different types are also of major significance.

Three single‐reference coupled‐cluster single and double (CCSD) substitution models, CCSD + T(CCSD), CCSDT‐1, and CCSD(T), exact through the fourth order of perturbation theory, are tested on the triplet ground‐state dissociation potential of O2. For the unrestricted Hartree‐Fock (UHF) initial approximation, the interatomic distance region R˜ 2.4–2.7 a.u. is problematic due to large multireference character, and beyond R˜ 3.0 a.u. spin contamination causes difficulties. To a large extent these deficiencies are cured by the coupled‐cluster treatments. Fourth‐order triples as well as higher‐order singles and doubles play a significant role. A more flexible basis set than DZP is important, too. None of the methods tried reproduce the experimental potential curve to the desired accuracy (2 kcal/mol). However, in a 6–311 + + G(2df, 2p) basis, the CCSD(T) model does give spectroscopic constants that are correct within 0.2 eV for the dissociation energy De, 70 cm−1for the harmonic vibrational frequency ve, and 0.5 cm−1for the first anharmonicity constant veXe. This model also reproduces the potential throughout the dissociative region to better than 0.2 eV. Overall CCSD + T(CCSD) is considerably less satisfactory than CCSD(T); CCSDT‐1 is similar to CCSD(T) in a DZP basis, but large basis calculations proved to be too time consuming.

The torsional satellite spectrum associated with the J=0→1 rotational transition in several symmetric isotopes of methylisilane is analyzed to yield the internal rotation barrier derivatives ∂V3/∂S1, where S1 is an A1 vibrational symmetry coordinate. Small basis set Hartree‐Fock calculations of the same quantities give remarkable agreement with the ``experimental'' values. But we have been unable to construct any simpler model that will reproduce the derivatives corresponding to either the methyl or silyl bend or the CSi stretch. The shielded proton model‐and variants which take into account the leftover negative charge as well as non‐Coulombic forces‐have been tried without success. From the derivatives we calculate a reduction in V3 of 54 cal/mole due to distortion of the equilibrium geometry in the eclipsed configuration. The effect of zero‐point motions on the observed barrier height is examined for the first time. It is estimated that the pure harmonic terms arising from just the A1 vibrations contribute about 155 cal/mole, and it is quite possible that the total correction could be 3 or 4 times larger.