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The results of CASSCF/multireference contracted CI calculations with large atomic natural orbital (ANO) basis sets are presented for selected regions of the N2H2 potential surface. The calculations focused on the addition of a H atom to HN2. This process involves four channels. Approach of the H atom along the HN bond leads to N2+H2 with no barrier (H abstraction), while approach of the H atom to the other three positions leads to trans-HNNH, cis-HNNH, and NNH2, which are stable minima. These calculations provide the most accurate energetics for these three stable species obtained to date. Combining the results of these calculations with previous results for HN2 provides accurate thermochemical data on these important molecules. [ABSTRACT FROM AUTHOR]

The results of an SCF/SDCI treatment are presented for selected portions of the ground state potential energy surface for the Cu3 molecule. For equilateral triangle geometries (D3h) the lowest state is 2E’ arising from 4sa’214se’1. The 2E’ state exhibits strong Jahn–Teller distortion, leading to 2A1 (acute angle) and 2B2 (obtuse angle) minima in C2v symmetry. Here the 2B2 minimum is a true minimum on the surface while the 2A1 minimum is a saddle point or very shallow secondary minimum connecting adjacent 2B2 minima. This strong Jahn–Teller distortion is consistent with the observed ground state vibrational levels in the gas phase spectroscopic studies of Morse, Hopkins, Langridge-Smith, and Smalley and in the matrix Raman studies of Moskovits. The 2B2 minimum is also consistent with the observed ESR spectrum of Cu3 in a matrix, which has been interpreted as an obtuse angle structure with most of the spin density on the end Cu atoms. A linear 2Σ+u state is found to be 0.26 eV higher. Two possible candidates have been found for the upper state in the spectrum of Morse et al. (i) a 3s Rydberg state of 2A1 symmetry and (ii) a 3d→4s state of 2E‘ symmetry. Both of these states are consistent with the observed selection rules. The 2E‘ state would be expected to be weakly Jahn–Teller distorted in agreement with the fit to the upper state levels by Morse et al. and by Thompson, Truhlar, and Mead. [ABSTRACT FROM AUTHOR]

We present a theoretical study of the bond dissociation energies (D0) for H2O and CH3OH. The C–H and O–H bond energies are computed accurately with the modified coupled-pair functional (MCPF) method using a large basis set. For these bonds, an accuracy of ±2 kcal/mol is achieved, which is consistent with the C–H and C–C single bond energies of other molecules. The C–O bond is much more difficult to compute accurately because it requires higher levels of correlation treatment and more extensive one-particle basis sets. [ABSTRACT FROM AUTHOR]