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The very good performance of modern density functional theory for molecular geometries and harmonic vibrational frequencies has been well established. We investigate the performance of density functional theory (DFT) for quartic force fields, vibrational anharmonicity and rotation–vibration coupling constants, and thermodynamic functions beyond the RRHO (rigid rotor–harmonic oscillator) approximation of a number of small polyatomic molecules. Convergence in terms of basis set, integration grid and the numerical step size for determining the quartic force field by using central differences of analytical second derivatives has been investigated, as well as the performance of various exchange-correlation functionals. DFT is found to offer a cost-effective approach with manageable scalability for obtaining anharmonic molecular properties, and particularly as a source for anharmonic zero-point and thermal corrections for use in conjunction with benchmark ab initio thermochemistry methods. [ABSTRACT FROM AUTHOR]

A new global, ground-state, Born-Oppenheimer surface is presented for the H[sup +] [sub 3] system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45000cm[sup -1] above the H[sup +] [sub 3] minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753). [ABSTRACT FROM AUTHOR]

In 1987, Seiler et al. (1987, Angew. Chem. Int. Edn Engl., 26, 1175) reported the observation of an eclipsed Csp3-CH3 bond in the low-temperature X-ray analysis of a crystalline trihydrate tricyclic orthoamide. This contrasts with the observed staggered conformation in an anhydrous crystal of the tricyclic orthoamide. Modest quantum chemical calculations estimated that a 5.5 kcal-mol-1 energy difference between the eclipsed form and the expected staggered form could be accounted for by unprecedented hydrogen bonding interaction strengths due to cooperative effects. In 1993, Novoa et al. (1993, Angew. Chem. Int. Edn Engl, 32, 588) showed that the earlier calculations were in error since they did not correct for basis set superposition error (BSSE). The BSSE corrected results of Novoa et al. could not account for the necessary 5.5 kcal mol-1 and this led them to question the original X-ray analysis of Seiler et al. Both sets of calculations suffered from the use of insufficiently accurate levels of theory and an inappropriate choice of model system. A current best estimate for the barrier to rotation is 1.4 kcal mol-1, far below that of the earlier studies. The stabilization of the eclipsed geometry due to hydrogen bonding after correction for BSSE is enough to match this small barrier. At today's best level of theory, the experimentally observed structure is 0.2 kcal mol-1 more stable than the staggered geometry. [ABSTRACT FROM AUTHOR]