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The accuracies of excitation energies of the cytosine molecule in the gas phase and in the aqueous solution calculated with noniterative and iterative coupled-cluster methods are discussed. It is shown that the inclusion of triply excited configurations has played a vital role in a proper description of the excitation energies of the cytosine molecule in realistic environment.

The article discusses the characterization of the signature absorption spectrum of a solvated hydroxyl OH radical as a solvent-to-solute charge transfer state. The intensity of such transfer is found to be dependent on the alignment of the molecular orbitals.

An asymptotic extrapolation scheme, based on the behavior of the electronic energies as functions of the cutoff factor for orbital energies corresponding to virtual orbitals is performed for the excited state large-scale calculations. The traditional coupled cluster (CC) method, in combination with the classical molecular dynamics is used to demonstrate the effects of the native environment and the dynamic fluctuations on the various biochemical systems, used frequently.

The completely renormalized [CR-CCSD (T)] method has been used to calculate the entire ground-state potential energy surface` (PES) for the Be + HF reaction. The study concluded that the single-reference 'black-box' CR-CCSD(T) method employing the spin- and symmetry-adapted RHF reference provides a highly accurate PES of the BeFH system, which is almost identical to the PES resulting form, the more expensive and more complicated MRCL(Q) calculations.

The theoretical results of the coupled-cluster, renormalized coupled cluster, and vibrational study on the trans-HNOO molecule are reported. The findings on HNOO are compared with the experimental results obtained by Ling, Boldyrev, Simons, and Wight (LBSW) and Laursen, Grace, DeKock, and Spronk (LGDS).

In this paper, we apply equation-of-motion coupled cluster (EOM-CC) methods in the studies of the vertical ionization potentials (IPs) and electron affinities (EAs) for a series of single-walled carbon nanotubes (SWCNT). The EOM-CC formulations for IPs and EAs employing excitation manifolds spanned by single and double excitations (IP/EA-EOM-CCSD) are used to study the IPs and EAs of the SWCNTs as a function of the nanotube length. Several armchair nanotubes corresponding to C 20n H 20 models with n = 2-6 have been used in benchmark calculations. In agreement with previous studies, we demonstrate that the electronegativity of C 20n H 20 systems remains, to a large extent, independent of the nanotube length. We also compare IP/EA-EOM-CCSD results with those obtained with coupled cluster models with single and double excitations corrected by perturbative triples, CCSD(T), and density functional theory (DFT) using global and range-separated hybrid exchange-correlation functionals.

In this paper we discuss the application of novel parallel implementation of the coupled cluster (CC) and equation-of-motion coupled cluster methods (EOMCC) in calculations of excitation energies of triplet states in β-carotene. Calculated excitation energies are compared with experimental data, where available. We also provide a detailed description of the new parallel algorithms for iterative CC and EOMCC models involving singles and doubles excitations.

Ab initio electronic structure theory calculations on cluster models support the characterization of the signature absorption spectrum of a solvated hydroxyl OH radical as a solvent-to-solute charge transfer state modulated by the hydrogen-bonding environment. Vertical excited states in OH(H2O)n clusters (n = 0-7, 16) calculated at the TDDFT level of theory (with companion calculations at the EOM-CCSD level of theory for n