Calculations are reported on the rotation-vibration energy levels of ammonia with associated transition intensities. A potential energy surface obtained from coupled cluster CCSD(T) calculations and subsequent fitting against experimental data is further refined by a slight adjustment of the equilibrium geometry, which leads to a significant improvement in the rotational energy level structure....

Auger decay is a relaxation process of core-vacant states in atoms and molecules, which one valence electron fills the core vacancy while second emitted. These pose challenge to electronic-structure theory, because they are embedded ionisation continuum. Recently, we showed that molecular can be described using complex-variable coupled-cluster (CC) methods partial widths branching ratios comput...

A four-dimensional potential energy surface (PES) for the CO dimer consisting of rigid molecules has been calculated, using a scheme that combines density functional theory to describe the monomers and symmetry adapted perturbation theory for the interaction energy (DFT-SAPT). The potential is fitted in terms of analytic functions, and the fitted potential is used to compute the lowest rovibrat...

The ground state vinylidene–acetylene isomerization was investigated by ab initio molecular electronic structure theory. The coupled-cluster method with single, double, and noniterative inclusion of triple excitations @CCSD~T!#; with single, double, and noniterative inclusion of triple and quadruple excitations @CCSD~TQ!#; and with full single, double, and triple excitations ~CCSDT! were used t...

A benchmark ab initio study on the thermochemistry of the trans-HNNH, cisHNNH, and H2NN isomers of diazene has been carried out using the CCSD(T) coupled cluster method, basis sets as large as [7s6p5d4f3g2h/5s4p3d2f1g], and extrapolations towards the 1-particle basis set limit. The effects on innershell correlation and of anharmonicity in the zero-point energy were taken into account: accurate ...

High-level quantum-chemical calculations are reported at the MP2 and CCSD(T) levels of theory for the equilibrium structure and the harmonic and anharmonic force fields of diacetylene, H−C ≡ C−C ≡ C−H. The calculations were performed employing Dunning’s hierarchy of correlation-consistent basis sets cc-pVXZ, ccpCVXZ, and cc-pwCVXZ, as well as the ANO2 basis set of Almlöf and Taylor. An empirica...

Neutral and anionic 13-atom aluminum clusters are studied with high-level, fully ab initio methods: second-order perturbation theory (MP2) and coupled cluster theory with singles, doubles, and perturbative triples (CCSD(T)). Energies and vibrational frequencies are reported for icosahedral and decahedral isomers, and are compared with density functional theory results. At the MP2 level of theor...

A reformulation of the perturbative triples correction to coupled cluster singles and doubles ~CCSD! based on the numerical Laplace transform of the energy denominator is presented. Rearranged equations reduce the O(N7) canonical scaling to O(N6), where N is a size measure of the electronic system. Two to three quadrature points is adequate for chemical predictions. The Laplace ansatz permits s...

Substituent effects on the carbon–carbon bonds of hydrocarbons have been a topic interest within past seven decades as resultant information would enable one to tune activity CC bonds. However, current assessments C≡C, C=C, and C−C bond strength acetylene, ethylene, ethane well their derivatives rely indirect measures such length dissociation enthalpy. In this work, we introduce quantitative me...