Psi4 is an ab initio electronic structure program providing methods such as HartreeFock, density functional theory, configuration interaction, and coupled-cluster theory. The 1.1 release represents a major update meant to automate complex tasks, such as geometry optimization using complete-basis-set extrapolation or focal-point methods. Conversion

The hydrogen bonding behavior of a homologous series of small molecule and polymeric urethanes, thiourethanes, and dithiourethanes was investigated in solution, melt, and solid states. The relative hydrogen bonding strengths in both small molecule and polymer systems were evaluated, and the results were compared to theoretical calculations of hydrogen bonding strength. The results for NMR and FTIR analysis of the small molecule models indicated that the NH protons on the carbamate and thiocarbamates have greater hydrogen bonding strengths than the NH protons of the dithiocarbamate. The polyurethane and polythiourethanes were found to have approximately equivalent physical and mechanical properties as a result of a similar extent of hydrogen bonding, whereas the polydithiourethane, due to a lower degree of hydrogen bonding, has reduced thermal and mechanical transition temperatures as well as lower hardness values. The polythiourethane and polydithiourethane networks exhibit narrower glass transitions compared to polyurethane networks, apparently the result of an efficient isocyanate/isothiocyanate-thiol reaction with little or no side products. Because of weakness of the C-S bond compared to the C-O bond, thiourethanes and dithiourethanes have lower thermal stability than the corresponding urethanes. Finally, the polythiourethanes and polydithiourethane have higher refractive index values than their polyurethane counterpart.

We have investigated the performance of the reduced-scaling coupled cluster method based on projected atomic orbitals (PAOs), pair natural orbitals (PNOs), and orbital specific virtuals (OSVs) for the prediction of linear response properties. These methods introduce different degrees of controllable sparsity in the ground-state and perturbed coupled cluster wave functions, leading to localization errors in properties such as dynamic polarizabilities and specific optical rotations. Using a series of chiral test compounds, we find that the inherent costs associated with computing response properties are significantly greater than those for determining the ground-state energy. As the dimensionality of the molecular system increases-from (pseudo)linear structures, such as fluoroalkanes, to cagelike structures, such as β-pinene-the crossover point between canonical-orbital and localized-orbital algorithms increases substantially. Furthermore, both the OSV and PNO methods provide greater reduction in cost (as measured by the size of the double-excitation space) than do PAOs, and PNOs provide the greatest level of sparsity for the systems examined here. Single-excitation truncation induces much larger errors than corresponding doubles truncation due to the fact that the first-order contribution to the one-electron perturbed wave function appears in the singles amplitudes. Both the PNO and OSV methods perform reasonably well for frequency-dependent polarizabilities provided appropriate thresholds are used for the occupation-number and weak-pair cutoffs on which each method depends. Specific rotations, however, are very sensitive to wave function truncation, to the extent that aggressive thresholds can yield the incorrect sign of the rotation, due to the delicate balance of positive and negative wave function contributions to the mixed electric-/magnetic-field response.

The impact of orbital localization on the efficiency and accuracy of the optimized-orbital coupled cluster model is examined for the prediction of chiroptical properties, in particular optical rotation. The specific rotations of several test cases-(P)-[4]triangulane, (S)-1-phenylethanol, and chiral conformers of 1-fluoropentane, heptane, and nonane-were computed using an approach in which localization is enforced throughout the orbital optimization and subsequent linear response computation. This method provides a robust local-correlation scheme for future production-level implementation. Although the cross-over point between the canonical and localized coupled cluster approach lies at larger molecules than for ground-state energies, the scheme presented should still provide reduced scaling sufficient to investigate much larger molecules than are presently accessible.