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We have investigated the dependence of the static first hyperpolarizability on the bond-length alternation (BLA) parameter. Our analysis indicates that the validity of the first hyperpolarizability/BLA parameter relationship is restricted to the no-field, vacuum, limit, while it successively breaks down along with increasing polarity of a surrounding medium, becoming invalid, for instance, in an aqueous solution. This contention is based on a series of TD-DFT, TD-DFT/PCM and hybrid TD-DFT/MM calculations of the first hyperpolarizability for a set of molecular configurations generated from Car--Parrinello hybrid QM/MM simulations of the stilbazolium merocyanine chromophore in chloroform and water solvents, and on a rationalization by means of the two-state model for the first hyperpolarizability. The BLA dependence of the three parameters entering the two-state model is shown to be qualitatively different in vacuum and in solvents. Particularly, in the vacuum case, the difference between ground and excited state dipole moments goes to zero for a vanishing BLA, which is not true in the presence of an aqueous medium. In the aqueous medium, an opposing behavior of the parameters involved in the two-state model results in an almost constant first hyperpolarizability with varying BLA parameter. bond-length alternation | hybrid QMMM modeling | nonlinear optical materials doi/ 10.1073/pnas.1006572107

Coupled cluster models and density functional theory methods used to calculate vibrational contributions to gas-phase specific rotation at 355, 589.3, and 633 nm reveal that vibrational contributions are crucial for understanding experimental result. Results show vibrational effects must be considered, when single-point calculations of specific optical rotations are used with experimentally determined specific optical rotations to resolve stereochemistry of chemical compounds with small-angle optical rotations.

The combined linear response coupled cluster/molecular mechanics (CC/MM) scheme including mutual polarization effects in the coupling Hamiltonian is applied together with supermolecular CC methods to the study of the gas-to-aqueous solution blue shift of the n-> excitation energy in acetone. The importance of including explicit polarization in both the molecular dynamics simulation as well as the CC/MM calculations is emphasized.

The calculations obtained from the combined quantum mechanics and molecular mechanics (QM/MM) method and general considerations related to the excited states of pentacene are used to propose an alternative where the intially excited species undergoes internal conversion to a doubly excited exciton. The combined approaches are supported by the observed photophysical properties of pentacene from both static as well as time-resolved experiments.

The article presents a study on the pH dependence of the activity and stability of a set of family 11 xylanases. The findings suggest that the thermostability of xylanases can be increased by mutations that introduce ionizable residues with p[K.sub.a] values significantly lower than the standard values.

Vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI) wave functions are used for describing the methods for automatic computation of infrared intensities and Raman activities. The inclusion of effects due to anharmonicity in the potential energy and property surfaces has improved the results remarkably when compared to experimental data.

A comparative study was conducted to determine the solvent effects on the [15.sup.N] NMR shielding constants and the lowest electronic excitation energy in the three diazines (pyrazine, pyrimidine, and pyridazine) in aqueous solution. The findings revealed that the potential deficiencies in the force field should be tested to ensure efficient hydrogen bonding in a consistent manner.

A theoretical study which focuses on the excited states of glycine, alanine and the related substructures formic acid, acetic acid, propionic acid, ammonia, methylamine, and ethylamine is presented. Also, calculations on electronic spectra for a number of related molecules, corresponding to the amino acid subgroups consisting of carboxylic acids and amines are presented.

The study presents linear response properties and excitation energies of liquid water calculated using different molecular mechanic approaches within coupled cluster theory. The study concluded that the CC2/MM response properties and excitation energies deviate somewhat from the corresponding CCSD/MM results.

A study of n->pie electronic transition in aqueous microsolvated formaldehyde using the coupled cluster (CC) and coupled cluster/molecular mechanics (CC/MM) methods is presented. The study concluded that the CC/MM method is a cost-efficient and reliable alternative to supermolecular calculations for the study of solvent effects on n-> electronic excitation energy of formaldehyde.

The solvent effects on the rotatory strength tensor are examined. The significant influence of correlation and solvent effects on the transition properties is discussed.

Theory and implementation of the first method that combined cluster and molecular mechanics (CC/MM) theory is presented. Interaction and solvation energies of condensed phase molecules were calculated by introducing the interactions between the solute (QM) and the solvent (MM) molecules into the coupled cluster Lagranfian.