Multireference Ab Initio Methods Research Articles

First-Principles Investigation of the Boron and Aluminum Carbides BC and AlC and Their Anions BC- and AlC-. 1

Using ab initio multireference methods and large correlation consistent basis sets, we have investigated the ground electronic structure of the carbides BC and AlC, the ground and the first two excited states of the corresponding anions, BC- and AlC-, and the ground (linear) structures of the hydrides H−BC and H−AlC. By employing a series of increasing size basis sets for the BC molecule, i.e., cc-pVnZ, aug-cc-pVnZ, cc-pCVnZ, and aug-cc-pCVnZ, n = 2, 3, 4, and 5, we have examined the convergence of its properties as a function of n. For both the neutral diatomic species and their anions we have obtained full potential energy curves, bond distances (re), dissociation energies (De), and the usual spectroscopic constants. For the BC molecule, our best re and De values are re = 1.4911 A and De = 102.2 kcal/mol in excellent agreement with experimental results. In the AlC case the calculated De = 77.13 kcal/mol is at least 12 kcal/mol higher than the experimental number. No experimental or theoretical data exis...

Theoretical Investigation of Scandium Carbide, ScC

The electronic structure of the diatomic carbide ScC has been investigated by ab initio multireference methods in conjunction with large basis sets. The ground state has been determined to be of 2Π symmetry with De = 65.5 kcal/mol and Re = 1.988 Å. Eight more states have been examined within an energy range of about 1 eV, with the first excited state (4Π) being 1.2 kcal/mol higher.

The Electronic Structure of ScAl+. Ground and Low-Lying Excited States

Using semiquantitative basis sets and ab initio multireference methods, we have investigated the electronic structure of scandium aluminide cation, ScAl+. In addition to the ground state (X2Δ), we ...

Diradicaloids: Description by the Spin-Restricted, Ensemble-Referenced Kohn−Sham Density Functional Method

A Kohn−Sham-type computational scheme capable of treating systems with strong nondynamic correlation is presented. The scheme, dubbed the spin-restricted, ensemble-referenced Kohn−Sham (REKS) method, is based on the representation of the density and energy for a strongly correlated system as weighted sums of densities and energies of several Kohn−Sham (KS) determinants. An optimal set of orthonormal KS orbitals and occupation numbers is obtained by minimizing the ground-state energy as a function of the density. Results of REKS calculations are reported and cover the following chemically important situations: (1) avoided crossing of potential energy surfaces, (2) bond-breaking processes, and (3) electronic structure of diradicals. The results of REKS calculations are compared with the available Kohn−Sham solutions for cases in which the exact density is known, as well as with results of conventional multireference ab initio methods and with the currently available density functional approaches.

Thermal Rearrangements of Norcaradiene

Multireference ab initio methods and density functional theory with a 6-31G* basis set have been applied to study the interconversions of norbornadiene, 1,3,5-cycloheptatriene, norcaradiene, and toluene. These include the [1,5]hydrogen shift in cycloheptatriene, the [1,5]carbon shift (walk rearrangement) in norcaradiene, the ring flip of cycloheptatriene, and the formation of norcaradiene from cycloheptatriene. Our best theoretical calculations for the walk rearrangement predict that the Woodward−Hoffmann “forbidden” suprafacial-inversion transition state and the “allowed” suprafacial-retention pathway differ in energy by <1 kcal/mol. Further, both transition states are effectively biradical-like. The transition state for formation of toluene from norcaradiene proceeds via a hydrogen transfer transition state that is formed directly from the “allowed” transition state for the walk rearrangement but not from the “forbidden” transition state. Further, the transition state for the transannular hydrogen shift...