The PSI4 program is a new approach to modern quantum chemistry, encompassing Hartree-Fock and density-functional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit high-efficiency computations of both standard and emerging electronic structure methods on conventional and high-performance parallel computer architectures. PSI4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the program's capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, PSI4 is available through an open-source license to the entire scientific community.

PSI3 is a program system and development platform for ab initio molecular electronic structure computations.The package includes mature programming interfaces for parsing user input, accessing commonly used data such as basis-set information or molecular orbital coefficients, and retrieving and storing binary data (with no software limitations on file sizes or file-system-sizes), especially multi-index quantities such as electron repulsion integrals. This platform is useful for the rapid implementation of both standard quantum chemical methods, as well as the development of new models. Features that have already been implemented include Hartree-Fock, multiconfigurational self-consistentfield, second-order Møller-Plesset perturbation theory, coupled cluster, and configuration interaction wave functions. Distinctive capabilities include the ability to employ Gaussian basis functions with arbitrary angular momentum levels; linear R12 second-order perturbation theory; coupled cluster frequency-dependent response properties, including dipole polarizabilities and optical rotation; and diagonal Born-Oppenheimer corrections with correlated wave functions. This article describes the programming infrastructure and main features of the package. PSI3 is available free of charge through the open-source, GNU General Public License.

Accurate prediction of higher-level electronic structure energies for large databases using neural networks, Hartree-Fock energies, and small subsets of the database Energy-switching potential energy surface for the water molecule revisited: A highly accurate singled-sheeted formThe global minimum on the water trimer potential energy surface has been investigated by means of second-order Mo "ller-Plesset ͑MP2͒ perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ ͑X ϭ D, T, Q, 5, 6͒, the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82Ϯ0.05 kcal mol Ϫ1 is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08 kcal mol Ϫ1 , and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9Ϯ0.2 kcal mol Ϫ1 . A zero-point vibrational correction of Ϫ5.43 kcal mol Ϫ1 has been computed from aug-cc-pVTZ MP2 harmonic vibrational frequencies. The accuracy of different computational schemes for obtaining the binding energies of the water dimer and trimer has been investigated, and computationally feasible methods are suggested for obtaining accurate structures and binding energies for larger water clusters.

group become more positive in (Z)-Id compared to toluene, while the N of the cyano group in (Z)-lc behaves in a similar manner when compared to that of acetonitrile.The calculations are in reasonable agreement with the NMR studies7 that indicate benzohydroxamic acid to exist predominantly in the (Z)-l form, while monoalkylhydroxamic acids exist in several forms. Hopefully, the data presented here will encourage the further work necessary to provide a more appropriate comparison. ConclusionThe ab initio results clearly show that Hartree-Fock calculations are not sufficient for describing the conformational preferences of formohydroxamic acid. At the highest levels optimized with MP2/6-31G(d,p) (£)-la is predicted to be most stable, but two other conformations ((Z)-la, and (Z)-2a) are so similar in energy that careful experimental work might be needed to accurately determine the structural preference of this molecule in the gas phase. The presence of substituents on the carbon of formohydroxamic acid is predicted to strongly affect the structural preferences of the acids. Therefore, experiments performed on the substituted acids may not be relevant to the parent.

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