Calculated using coupled-cluster

Harmonic and cubic force fields of Sis were calculated using coupled-cluster (CC) theory (25) and a correlation-consistent basis set. Specifically, the CC singles and doubles (CCSD) method (24) was used in conjunction with the cc-pVTZ basis set (25) developed by Dunning and co-workers. The force... 

Tel. 904-392-1597, fax 904-392-8722, e-mail bartlett qtp.ufl.edu Ab initio molecular orbital calculations using coupled-cluster and many-body perturbation theory methods. 

One can avoid these problems by using Coupled Cluster (CC) theory107, which contains infinite-order effects and therefore does not lead to the oscillatory behaviour of properties calculated with MPn108. Homoaromatic stabilization energies have been calculated for smaller molecules with CCSD(T) or QCISD(T)54 56. These are CC methods, which cover S and D excitations and, in addition, include T effects in a perturbational way109110. They represent some of the most accurate single determinant ab initio methods available today that can be applied in a routine way. 

There have been many calculations of a for neon itself at various levels of approximation. Some of these are summarized in Ref. [26] and a basis set analysis has been made in Ref. [25]. Of the electron-correlated results[42]-[47], there appears to be a consensus that the best value using coupled-cluster-single-doubles wavefunctions with a perturbational estimate of connected triplet excitations, CCSD(T), is 2.680-2.690 au[25],[44],[46],[47]. 

In the present paper, we show that it is possible to calculate both vibrational and electronic transitions of H2SO4 with an accuracy that is useful in atmospheric simulations. We calculate the absorption cross sections from the infrared to the vacuum UV region. In Section 2 we describe the vibrational local mode model used to calculate OH-stretching and SOH-bending vibrational transitions as well as their combinations and overtones [42-44]. This model provides frequencies and intensities of the dominant vibrational transitions from the infrared to the visible region. In Section 3 we present vertical excitation energies and oscillator strengths of the electronic transitions calculated with coupled cluster response theory. These coupled cluster calculations provide us with an accurate estimate of the lowest... 

Given that the electronic structure of H2SO4 is well described by a single Slater determinant and does not have significant multi reference character, an alternative approach is to use coupled cluster (CC) response functions to calculate the electronic transitions [76]. We will show in this section that CC calculations can provide calculated electronic transitions in H2SO4 and thus a cross section up to and including the Lyman-a region. 

Molecular polarizabilities and hyperpolarizabilities are now routinely calculated in many computational packages and reported in publications that are not primarily concerned with these properties. Very often the calculated values are not likely to be of quantitative accuracy when compared with experimental data. One difficulty is that, except in the case of very small molecules, gas phase data is unobtainable and some allowance has to be made for the effect of the molecular environment in a condensed phase. Another is that the accurate determination of the nonlinear response functions requires that electron correlation should be treated accurately and this is not easy to achieve for the molecules that are of greatest interest. Very often the higher-level calculation is confined to zero frequency and the results scaled by using a less complete theory for the frequency dependence. Typically, ab initio studies use coupled-cluster methods for the static values scaled to frequencies where the effects are observable with time-dependent Hartree-Fock theory. Density functional methods require the introduction of specialized functions before they can cope with the hyperpolarizabilities and higher order magnetic effects. 

Working to similar levels of accuracy, Pawlowski et al have calculated the static and frequency-dependent linear polarizability and second hyperpolarizability of the Ne atom using coupled-cluster methods with first order relativistic corrections. Good agreement with recent experimental results is achieved. Klopper et al.s have applied an implementation of the Dalton code that enables... 

Rizzo and Coriani233 have used coupled cluster ab initio methods to calculate the Jones birefringence of He, Ne, Ar and Kr (and also for H2, N2, C2H2 and CO) and compare the values with the birefringence induced by the Cotton-Mouton effect. They find that the latter is between 100 and 3500 times larger than the former. 

With regard to the electronic structure methodology, major obstacles must be surmounted before improvements can be made. Calculations with Coupled-Cluster methods, an obvious next step, are far more computationally costly than the presently used MP2, or B3LYP methods. In fact, there are extremely few direct ab initio calculations of anharmonic vibrational spectroscopy at higher than MP2 or DPT levels, even for small polyatomics. From the point of view of ab initio anharmonic spectroscopy, the leap from MP2 to the Coupled-Cluster method seems a bottleneck. One can draw encouragement from faster Coupled-Cluster implementations, so far employed with the perturbation theory anharmonic analysis [116,117]. 

Janowskl, T, Ford, A. R., and Pulay, P. (2007). Parallel calculation of coupled cluster singles and doubles wave functions using array files, / Chem. Theory Comput 3, pp. 1368-1377, doi 10.1021/ct700048u. 

Calculation of Intermolecular Interactions in the Benzene Dimer Using Coupled-Cluster and Local Electron Correlation Methods. 

Correlated calculations, such as configuration interaction, DFT, MPn, and coupled cluster calculations, can be used to model small organic molecules with high-end workstations or supercomputers. These are some of the most accurate calculations done routinely. Correlation is not usually required for qualitative or even quantitative results for organic molecules. It is needed to obtain high-accuracy quantitative results. 

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