Michael Hanrath

An exponential multireference wave-function Ansatz

An exponential multireference wave-function Ansatz is formulated. In accordance with the state universal coupled-cluster Ansatz of Jeziorski and Monkhorst [Phys. Rev. A 24, 1668 (1981)] the approach uses a reference specific cluster operator. In order to achieve state selectiveness the excitation- and reference-related amplitude indexing of the state universal Ansatz is replaced by an indexing which is based on excited determinants. There is no reference determinant playing a particular role. The approach is size consistent, coincides with traditional single-reference coupled cluster if applied to a single-reference, and converges to full configuration interaction with an increasing cluster operator excitation level. Initial applications on BeH2, CH2, Li2, and nH2 are reported.

Fully automated implementation of the incremental scheme: Application to CCSD energies for hydrocarbons and transition metal compounds

A general fully automated implementation of the incremental scheme for molecules and embedded clusters in the framework of the coupled cluster singles and doubles theory is presented. The code can be applied to arbitrary order of the incremental expansion and is parallelized in a master/slave structure. The authors found that the error in the total correlation energy is lower than 1 kcal/mol with respect to the canonical CCSD calculation if the incremental series is truncated in a proper way.

TWO NOVEL THERMAL BIRADICAL CYCLIZATIONS OF ENYNE-KETENIMINES : THEORY, EXPERIMENT, AND SYNTHETIC POTENTIAL

Both benzocarbazoles and quinolines can be synthesized from enyne ketenimines 1 generated in situ via biradical intermediates (see reaction below). Which of the heterocyclic ring systems is formed depends on the choice of the substituent R1 at the alkyne terminus.

Higher excitations for an exponential multireference wavefunction Ansatz and single-reference based multireference coupled cluster Ansatz: Application to model systems H4, P4, and BeH2

This article reports on the convergence of the exponential multireference wavefunction Ansatz (MRexpT) [J. Chem. Phys. 123, 84102 (2005)] and the single-reference based multireference coupled cluster Ansatz [J. Chem. Phys. 94, 1229 (1991)] with respect to higher cluster excitations. The approaches are applied to the H(4), P(4), and BeH(2) model systems according to the recently published analysis by Evangelista et al. [J. Chem. Phys. 125, 154113 (2006)]. The results show both MRexpT and SRMRCC to be highly accurate although SRMRCC shows problems due to its lack of Fermi vacuum invariance (symmetry breaking).

Two Novel Thermal Biradical Cyclizations in Theory and Experiment: New Synthetic Routes to 6H‐Indolo[2,3‐b]quinolines and 2‐Aminoquinolines from Enyne‐Carbodiimides

The regioselectivity of the biradical cyclization of enyne-carbodiimides 1 can easily be controlled by variation of R1 at the alkyne terminus. Attachment of a hydrogen atom (R1 =H) leads to C2 -C7 cyclization and formation of biradical 2, whereas C2 -C6 cyclization to provide biradical 3 is observed with R1 =Me3 Si or Ph.

Regioselectivity of Biradical Cyclizations of Enyne-Allenes: Influence of Substituents on the Switch from the Myers–Saito to the Novel C2–C6 Cyclization

The nature of the substituent at the alkyne terminus of enyne-allenes is critical for the switch from the Myers–Saito C2–C7 cyclization to the C2–C6 cyclization (see reaction scheme below). Quantum-chemical calculations show the influence of steric (R=tBu) and electronic effects (R=Ph) on the regioselectivity of the thermal reaction; for R=NH2 a reduction in activation energy to below 18 kcal mol−1 and a modified reaction mechanism for the C2–C6 cyclization are postulated. R1=R2=H.

Electronic absorption spectra of linear C6, C8 and cyclic C10, C12 in neon matrices

The electronic absorption spectra of the even-numbered carbon molecules C6–C14 have been measured in neon matrices. Bare carbon anions were produced in a cesium sputter source, mass selected, codeposited with neon at 6 K, and neutralized. The spectra show, apart from the known (1) 3Σu−←X 3Σg− transition of linear C6, C8, and C10 in the visible, absorption bands in the UV region. The spectral data when considered in conjunction with ab initio calculations show that the linear forms of C6 and C8 have the next strong (2) 3Σu−←X 3Σg− transition with band maximum near 238 and 277 nm, respectively, whereas the band systems of C10, C12, and C14 at 316, 332, and 347 nm are due to the monocyclic species.

Dissociating N2: a multi-reference coupled cluster study on the potential energy surfaces of ground and excited states

This article reports on the calculation of 12 low lying states of the nitrogen molecule along its dissociation using the multi-reference exponential wavefunction ansatz [J. Chem. Phys. 123, 84102 (2005)], the single-reference formalism multi-reference coupled cluster [J. Chem. Phys. 94, 1229 (1991)], and MRCI methods. Energies relative to full CI are given. The results show the multi-reference coupled cluster approaches generally applicable to very demanding problems at high accuracy. In comparison to MRCI both coupled cluster type approaches do not reproduce spatial symmetry exactly. Nevertheless, this error in the degeneracy is much less than the overall error of the MRCI calculations.

Evaluation of incremental correlation energies for open-shell systems: application to the intermediates of the 4-exo cyclization, arduengo carbenes and an anionic water cluster.

A fully automated procedure for incremental closed-shell CCSD calculations has been extended to open-shell cases tractable with the restricted open-shell CCSD method. It is demonstrated that for monoradical intermediates of the 4-exo cyclization, the triplet state of Arduengo carbenes as well as for water cluster anions chemical accuracy can be reached with respect to the error introduced by the local correlation treatment. Finally, it is shown that the computationally less demanding evaluation of higher-order increments in a smaller basis set does not lead to significant errors.

Fully Automated Implementation of the Incremental Scheme for Correlation Energies

Abstract The applicability and performance of a fully automated implementation of the incremental scheme for the evaluation of correlation contributions in wavefunction-based quantum chemical calculations on large molecules is briefly reviewed and a typical example from organic chemistry is discussed in more detail. The accuracy of relative energies is analyzed for various water hexamers at the CCSD(T)/aug-cc-pVDZ level. Furthermore the accuracy of the potential energy surface of octane is analyzed at the CCSD level of theory. Finally, CCSD(T)/aug-cc-pVTZ calculations with 1656 basis functions in C1 symmetry are reported for (H2O)18, where a full calculation is infeasible within reasonable time.