Augmented Correlation Consistent Basis Research Articles

Potential energy curves, turning points, Franck–Condon factors and $r$-centroids for the astrophysically interesting $\mbox{S}_{2}$ molecule

We have theoretically studied the seven lowest triplet states of the disulfur species. For such investigations, analytical potential energy curves (APECs) for $X^{3}\varSigma _{g}^{-}$ ground and $A^{\prime\,3} \Delta _{u}$, $A^{3}\varSigma _{u}^{+}$, $B^{\prime\prime\,3}\varPi _{u}$, and $B^{3}\varSigma _{u}^{-}$ excited electronic states were constructed within the extended Hartree–Fock approximate correlation energy (EHFACE) model by Varandas. Once that these analytical representations are obtained, nuclear properties, such as vibrational energies, classical turning points, and spectroscopic constants were calculated. Particularly, comparisons between these vibrational levels with those obtained via the Rydberg–Klein–Rees (RKR) methodology as implemented in Le Roy’s RKR1 code are reported. The impact of tight $d$ augmented correlation consistent basis on the energies and frequencies is shown. We also re-examined the vibronic (vibration-electronic) transition parameters as Franck–Condon (FC) factors and $r$-centroids for the bands of the $B^{\prime\prime\,3}\varPi _{u} $–$X^{3}\varSigma _{g}^{-}$, $B^{3}\varSigma _{u}^{-} $–$X^{3}\varSigma _{g}^{-}$, $C^{3}\varSigma _{u}^{-} $–$X ^{3}\varSigma _{g}^{-}$, and $D^{3}\varPi _{u} - X^{3}\varSigma _{g}^{-}$ systems of the $\mbox{S}_{2}$ molecule. The vibrational levels and turning points of the two Rydberg states $C^{3}\varSigma _{u}^{-}$ and $D^{3}\varPi _{u}$ were computed only with the RKR method. Our results can be employed in rationalizations of astrochemical and astrophysical observations.

A full-dimensional ab initio potential energy surface and rovibrational energies of the Ar–HF complex

ABSTRACTWe report a new full-dimensional ab initio potential energy surface for the Ar–HF van der Waals complex at the level of coupled-cluster singles and doubles with noniterative inclusion of connected triples levels [CCSD(T)] using augmented correlation-consistent quintuple-zeta basis set (aV5Z) plus bond functions. Full counterpoise correction was employed to correct the basis-set superposition error. The hypersurface was fitted using artificial neural network method with a root mean square error of 0.1085 cm−1 for more than 8000 ab initio points. The complex was found to prefer a linear Ar–H–F equilibrium structure. The three-dimensional discrete variable representation method and the Lanczos propagation algorithm were then employed to calculate the rovibrational states without separating inter- and intra- molecular nuclear motions. The calculated vibrational energies of Ar–HF differ from the experiment values within about 1 cm−1 on the first four HF vibrational states, and the predicted pure rotational energies on (0000) and (1000) vibrational states are deviated from the observed value by about 1%, which shows the accuracy of our new PES.

Weakly interacting molecular clusters of CO with H2O, SO2, and NO+

Intermolecular interactions in three dimers, CO···H2O, CO···SO2, and CO···NO+, were studied at the CCSD(T) level of theory, using a series of the augmented correlation consistent polarised basis sets. Interaction energy and its components as well as vibrational spectra for local minima were computed using both harmonic and anharmonic approximations. While CO···H2O and CO···SO2 are weakly bound with the binding energies −7.4 and −6.4 kJ/mol, CO···NO+ is much more stable with the binding energy of −32.8 kJ/mol corresponding to ΔG = −4.7 kJ/mol at 254 K.

Nuclear magnetic resonance shieldings of water clusters: is it possible to reach the complete basis set limit by extrapolation?

We have explored the possibility of using extrapolation techniques to estimate the complete basis set limit of nuclear shieldings of small water clusters. Several density functionals (Becke-Lee-Yang-Parr (BLYP), Perdew-Burke-Ernzerhof (PBE), OPTX-Perdew-Burke-Ernzerhof (OPBE), Swart-Solà-Bickelhaupt functional with Grimme's Dispersion included (SSB-D) and wavefunction methods (Hartree–Fock, Møller-Plesser perturbation theory for electron correlation up to second order (MP2) and coupled cluster) were used in combination with Dunning's (augmented) correlation-consistent and Jensen's polarisation-consistent basis sets. Although density functionals show larger nuclear shieldings than coupled cluster methods for these water clusters, differences in chemical shifts are much smaller. A new extrapolation scheme based on a Gaussian exponential function is shown to achieve better results for reaching the basis set limiting values, but the exponential needed for it cannot be obtained a priori with good confidence. This renders a two-point extrapolation for prediction of the basis set limiting value impossible.

BO3 Molecular Structures: Examples of the Importance of Electron Correlation

In 1991 Burkholder and Andrews reported the spectroscopic identification of the boron- oxygen species BO, BO2, B2O2, B2O3, and BO2-. In addition, they tentatively identified two infrared features due to BO3. In this research, a wide range of possible BO3 structures is considered theoretically. The highest level of theory used involves the CCSD(T) method with an augmented correlation consistent quadruple zeta basis set. A planar structure O-B-O-O is predicted to be the global minimum, lying 4.2 kcal mol-1 below a higher symmetry (C2v) structure incorporating a BO2 isosceles triangle. Reasonable agreement is found between the theoretical vibrational frequencies and the two fundamentals reported by Burkholder and Andrews. The potentially important ozone adduct B-O3 is predicted to lie much higher in energy.

Ab initio potential energy surface for HeF2 in its ground electronic state

The ground state potential energy surface for He-F 2 has been generated using the coupled-cluster singles and doubles excitation approach with perturbative treatment of triple excitations [CCSD(T)] and multi-reference configuration interaction (MRCI) methodologies, with augmented correlation consistent quadruple zeta basis set and diffused functions. Both the CCSD(T) and MRCI surfaces are compared and the results analyzed. The CCSD(T) surface exhibits van der Waals minima at different distances for different orientations of He approaching F 2 and is adequate to describe accurately only in the region around the equilibrium bond distance of F 2 . The MRCI surface, on the other hand, yields reliable results for a wider range of F-F bond distances leading to the correct asymptote. Davidson correction to the MRCI surface makes it purely repulsive over the regions investigated.

ChemInform Abstract: Stabilities of Hydrocarbons and Carbocations. Part 1. A Comparison of Augmented 6‐31G, 6‐311G, and Correlation Consistent Basis Sets.

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