David H. Magers

Bio: David H. Magers is an academic researcher from Harvard University. The author has contributed to research in topics: Diborane & Boranes. The author has an hindex of 7, co-authored 12 publications receiving 456 citations. Previous affiliations of David H. Magers include University of Florida.

Isomers and excitation energies of C4

Abstract: Coupled‐cluster (CC) and many‐body perturbation theory (MBPT) studies of the rhombic and linear structures of C4 are reported. For each isomer, the electronic spectra is obtained, and comparisons are made with experimental matrix‐isolated ESR and electronic spectra. The closed‐shell 1Ag rhombic ground state is found to be more stable than the 3∑−g state of the linear isomer by 5 kcal/mol at the highest level of calculation performed (CCSDT‐1). However, the predicted spectrum for linear C4 is in reasonable agreement with the observed results. An allowed electronic transition for the rhombus is predicted to lie in the same region, suggesting the possibility that both isomers could coexist in the experiment. Finally, vibrational frequencies for the rhombic isomer are calculated using analytical second‐order MBPT second derivatives to aid in the experimental identification of this transient species.

Highly correlated single‐reference studies of the O3 potential surface. I. Effects of high order excitations on the equilibrium structure and harmonic force field of ozone

Abstract: The potential energy surface of ozone in the vicinity of the equilibrium geometry is investigated by single‐reference many‐body perturbation theory (MBPT) and coupled‐cluster (CC) methods. As expected from the known inadequacies of the independent‐particle picture of O3, analysis of the CCSDT‐1 wave function reveals considerable mixing between the [core⋅⋅⋅]4b226a211a22 and [core⋅⋅⋅]4b226a212b21 configurations. Smaller, but still significant, contributions come from other configurations involving redistribution of electrons within the out‐of‐plane π orbital framework. As expected, the equilibrium structure and harmonic force field computed at the SCF level of theory are in considerable error. When allowance is made for electron correlation effects, the discrepancies between theory and experiment for the equilibrium structure and totally symmetric force field are significantly reduced, and the MBPT(4), CCSD, CCSD+T(CCSD) and CCSDT‐1 results are in reasonable agreement with accepted values. Asymmetric stretc...

Stability and properties of C4 isomers

Abstract: The relative electronic energies of the 1Ag rhombus and the 3Σ−g linear isomers of C4 have been computed using a 5s3p1d basis at various levels of coupled‐cluster and many‐body perturbation theory. At the highest level performed, CCSD+T(CCSD), the two isomers are essentially isoenergetic. Themodynamic results indicate, however, that the linear isomer will be significantly more abundant in the plasma in which C4 is formed. Vibrational frequencies are presented and compared with experimental results; some of the prior assignments are questioned. The isotropic ESR hyperfine splitting parameters for the linear triplet state have been computed and are also compared with experimental results. Finally, the ionization potential and electron affinity for both structures are presented.

Correlated studies of infrared intensities

Abstract: Coupled‐cluster and many‐body perturbation theories are applied to an investigation of infrared absorption intensities within the double‐harmonic approximation. In agreement with previous studies, both electron correlation and basis set dependencies are found to be significant, particularly for stretching vibrations which involve hydrogen atoms. Intensities calculated at the highly correlated CCSD+T(CCSD) level with large Gaussian basis sets are in reasonable agreement with experiment for HF and the ν2 and ν3 modes of water, while the intensity of ν1 is significantly overestimated even with a relatively large 53‐CGTO basis. In addition, intensities and harmonic frequencies calculated at the SCF and MBPT(2) levels with a double‐zeta plus polarization (DZP) basis set are presented for a number of first row compounds, and are compared to recent experimental values. Although agreement between experimental and SCF intensities is poor, these discrepancies are moderated considerably when correlation is introduce...

Coupled-cluster theory in quantum chemistry

Abstract: Today, coupled-cluster theory offers the most accurate results among the practical ab initio electronic-structure theories applicable to moderate-sized molecules. Though it was originally proposed for problems in physics, it has seen its greatest development in chemistry, enabling an extensive range of applications to molecular structure, excited states, properties, and all kinds of spectroscopy. In this review, the essential aspects of the theory are explained and illustrated with informative numerical results.

The equation of motion coupled‐cluster method. A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties

Abstract: A comprehensive overview of the equation of motion coupled‐cluster (EOM‐CC) method and its application to molecular systems is presented. By exploiting the biorthogonal nature of the theory, it is shown that excited state properties and transition strengths can be evaluated via a generalized expectation value approach that incorporates both the bra and ket state wave functions. Reduced density matrices defined by this procedure are given by closed form expressions. For the root of the EOM‐CC effective Hamiltonian that corresponds to the ground state, the resulting equations are equivalent to the usual expressions for normal single‐reference CC density matrices. Thus, the method described in this paper provides a universal definition of coupled‐cluster density matrices, providing a link between EOM‐CC and traditional ground state CC theory.Excitation energy,oscillator strength, and property calculations are illustrated by means of several numerical examples, including comparisons with full configuration interaction calculations and a detailed study of the ten lowest electronically excited states of the cyclic isomer of C4.

The full CCSDT model for molecular electronic structure

Abstract: The full coupled‐cluster model (CCSDT) single, double, and triple excitation method defined by the wave function exp(T1+T2+T3)‖Φ0〉 is formulated and computationally implemented for the first time. Explicit computational equations are presented. The method is applied to numerous examples including BH, FH, C2H2, CO, Ne, F−, and H2O to assess its applicability to the correlation problem. Results from CCSDT agree with full CI, to an average error of less than 1 kcal/mol even for difficult bond breaking examples. A series of results for various approximate, but computationally more efficient versions of the full CCSDT model are also studied and shown to give results in excellent agreement with CCSDT. Additional comparisons with fifth‐order MBPT are reported.