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The potential energy curves (PECs) and spectroscopic constants of the ground and excited states of a LiMg+ molecular cation were investigated. We obtained accurate results for the fifteen lowest-lying states of the LiMg+ cation using the Intermediate Hamiltonian Fock Space Multireference Coupled Cluster (IH-FS-CC) method applied to the (2,0) sector. Relativistic corrections were accounted for using the third-order Douglas–Kroll method. In each instance, smooth PECs were successfully computed across the entire range of interatomic distances from equilibrium to the dissociation limit. The results are in good accordance with previous studies of this molecular cation. Notably, this study marks the first application of IH-FS-CC in investigating a mixed alkali and alkaline earth molecular cation, proving its usability in determining accurate PECs of such diatomics and their spectroscopic constants. [ABSTRACT FROM AUTHOR]

Creation of descriptive metadata for digital objects tends to be a laborious process. Specifically, subject analysis that seeks to classify the intellectual content of digitized documents typically requires considerable time and effort to determine subject headings that best represent the substance of these documents. This project examines the use of topic modeling to streamline the workflow for assigning subject headings to the digital collection of New Mexico State University news releases issued between 1958 and 2020. The optimization of the workflow enables timely scholarly access to unique primary source documentation., INTRODUCTION Digital scholarship relies on digital collections and data. In the influential book Digital_Humanities, Anna Burdick and her associates affirm that humanistic knowledge production depends on collection building and curation. [...]

Several levels of theory, including both Gaussian-based and plane wave density functional theory (DFT), second-order perturbation theory (MP2), and coupled cluster methods (CCSD(T)), are employed to study Au6 and Au8 clusters. All methods predict that the lowest energy isomer of Au6 is planar. For Au8, both DFT methods predict that the two lowest isomers are planar. In contrast, both MP2 and CCSD(T) predict the lowest Au8 isomers to be nonplanar.

We consider classes of noniterative (∼n7) ΛCCSD(TQf) (coupled cluster singles and doubles with noniterative triples and quadruples) for bond breaking including approximations based on Λ2CCSD(TQf). All methods are applied to the standard tests for the treatment of potential energy curves for small molecules (HF, F2, H2O, N2, and C2) where unambiguous comparison to full configuration interaction exists. Depending on the nature of the electron correlation substantial improvements are obtained at the same ∼n7 cost over CCSD(T). [ABSTRACT FROM AUTHOR]

The recently reported inclusion of the connected triples into the intermediate Hamiltonian realization of the Fock space coupled-cluster (IH-FS-CC) theory [M. Musial and R. J. Bartlett, J. Chem. Phys. 129, 044101 (2008)] is extended to produce the triplet states. This is done entirely in spatial orbitals on the basis of the double occupancy in the restricted Hartree Fock reference function. New equations for the triplet state amplitudes expressed in terms of the Goldstone diagrams are derived and implemented. Several applications show the usefulness of the IH-FS-CC scheme to describe the triplet states with the computational gains inherent to a spin-free implementation. [ABSTRACT FROM AUTHOR]

The multireference Fock-space coupled-cluster (FS-CC) equations constructed via the effective Hamiltonian approach are reduced to those obtained through a partitioning technique for the matrix diagonalization. This allows finding a better route to solve the FS-CC equations for the (1,1) sector by the direct diagonalization of the properly constructed matrix, which is identical to that obtained with the intermediate Hamiltonian approach. The detailed connections with the equation-of-motion CC (EOM-CC) approach also provides alternative routes for studying the higher-sectors of Fock space, and for the treatment of properties other than the energy. The approach studied in this work is formulated at the CC singles, doubles (CCSD) and CC singles, doubles, triples (CCSDT) level. The performance of the method is illustrated by numerical examples for the C2, C2H4, and H2CO molecules in comparison with the EOM-CC method and available experimental data. [ABSTRACT FROM AUTHOR]

The intermediate Hamiltonian multireference coupled-cluster (CC) method with singles, doubles, and triples within the excited (1,1) sector of Fock space (FS) is implemented and formulated to calculate excitation energies (EEs). Due to the intermediate Hamiltonian formulation, which provides a robust computational scheme for solving the FS-CC equations, coupled to an efficient factorization strategy, relatively large basis sets and model spaces are employed permitting basis set converged comparisons of the calculated vertical EEs, which can be compared to the experimental data for the N2 and CO molecules. The issue of charge-transfer separability is also addressed. [ABSTRACT FROM AUTHOR]

We report the initial Fock space multireference coupled cluster method with the full inclusion of single, double, and triple excitations (FS-CCSDT) for the (1,1) sector. We present pilot applications for calculating excitation energies for the N2 molecule and the Ne atom. The performance of the current model, along with the FS-CCSD one, has been studied in comparison with the equation-of-motion coupled-cluster and the similarity transformed methods. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Calculated vertical excitation energies, optimized geometries, and vibrational frequencies of the nitric oxide dimer are reported. The “multireference” (MR) nature of the problem and weak bond between the monomers make a proper description of the system difficult, and standard methods are not as applicable to this system. In this study, recently developed methods such as the double-electron-affinity similarity-transformed equation-of-motion coupled cluster method (DEA-STEOM-CCSD), MR Brillouin–Wigner CCSD (MR-BWCCSD), MR average quadratic CCSD (MR-AQCCSD), and others are used along with a series of basis sets of increasing accuracy. The calculated excitation energies are consistent and convergent with respect to the basis set in DEA-STEOM-CCSD, MR-BWCCSD, and MR-AQCCSD methods. The geometries are highly sensitive to the basis set size and the challenge to obtain the right answers in the basis set limit remains. Nevertheless, we obtain qualitative agreement with the experimental geometry and harmonic vibrational frequencies. The results from the above multireference methods show dramatic improvement over the coupled cluster with singles and doubles and perturbative triples excitation [CCSD(T)] results. Like O[sub 3], (NO)[sub 2] offers an extremely challenging example in its ground and excited states for single-reference and multireference theory. It deserves to be a standard test molecule as new methods are developed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The aspects of the coupled-cluster theory which offers the most accurate results among the practical ab initio electronic-structure theories applicable to moderate-sized molecules is illustrated with informative numerical results. The theory, which was originally proposed for problems in physics, was observed to be very helpful in development of chemistry, for application of molecular structure, excited states, properties, and all kinds of spectroscopy.

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).

We formulate the theory for a diatomic molecule in a spatially degenerate electronic state interacting with a non-resonant laser field and investigate its rovibrational structure in the presence of the field. We report on ab initio calculations employing the double electron attachment intermediate Hamiltonian Fock space coupled cluster method restricted to single and double excitations for all electronic states of the Rb2 molecule up to 5s+5d dissociation limit of about 26,000 cm-1. In order to correctly predict the spectroscopic behaviour of Rb2, we have also calculated the electric transition dipole moments, non-adiabatic coupling and spin-orbit coupling matrix elements, and static dipole polarisabilities, using the multireference configuration interaction method. When a molecule is exposed to strong non-resonant light, its rovibrational levels get hybridised. We study the spectroscopic signatures of this effect for transitions between the X¹ Σg+ electronic ground state and the A¹Σu+ and b³πu excited state manifold. The latter is characterised by strong perturbations due to the spin-orbit interaction. We find that for non-resonant field strengths of the order 109 W/cm², the spin-orbit interaction and coupling to the non-resonant field become comparable. The non-resonant field can then be used to control the singlet-triplet character of a rovibrational level. [ABSTRACT FROM AUTHOR]