Micah L. Abrams

Full configuration interaction potential energy curves for the X 1Σg+, B 1Δg, and B′ 1Σg+ states of C2: A challenge for approximate methods

The C2 molecule exhibits unusual bonding and several low-lying excited electronic states, making the prediction of its potential energy curves a challenging test for quantum chemical methods. We report full configuration interaction results for the X 1Σg+, B 1Δg, and B′ 1Σg+ states of C2, which exactly solve the electronic Schrodinger equation within the space spanned by a 6-31G* basis set. Within the D2h subgroup used by most electronic structure programs, these states all have the same symmetry (1Ag), and all three states become energetically close for interatomic distances beyond 1.5 A. The quality of several single-reference ab initio methods is assessed by comparison to the benchmark results. Unfortunately, even coupled-cluster theory through perturbative triples using an unrestricted Hartree–Fock reference exhibits large nonparallelity errors (>20 kcal mol−1) for the ground state. The excited states are not accurately modeled by any commonly used single-reference method, nor by configuration interac...

Optical rotatory dispersion of 2,3-hexadiene and 2,3-pentadiene.

The specific rotation of (P)-2,3-hexadiene (1) was measured as a function of wavelength for the gas phase, the neat liquid, and solutions. There was a surprisingly large difference between the gas phase and condensed phase values. The specific rotation was calculated using B3LYP and CCSD, and the difference in energy between the three low energy conformers was estimated at the G3 level. The Boltzmann-averaged CCSD-calculated rotations using the gauge independent velocity gauge representation, as well as the B3LYP values, are in agreement with the gas-phase experimental values. In order to avoid possible problems associated with the conformers of 1, 2,3-pentadiene (2) also was examined. Here again, there was a large difference between the gas-phase and condensed-phase specific rotations, with the CCSD velocity gauge (and B3LYP) results being close to the gas-phase experimental values. The possibility that 2,3-pentadiene could be distorted on going from the gas to liquid phase, thereby accounting for the effect of phase on the specific rotation, was examined via a Monte Carlo statistical mechanics simulation. No effect on the geometry was found. Specific rotations of 1 found in solutions were similar to those for the liquid phase, indicating that the phase difference was not due to association.

A comparison of polarized double-zeta basis sets and natural orbitals for full configuration interaction benchmarks

We compare several standard polarized double-zeta basis sets for use in full configuration interaction benchmark computations. The 6-31G**, DZP, cc-pVDZ, and Widmark–Malmqvist–Roos atomic natural orbital (ANO) basis sets are assessed on the basis of their ability to provide accurate full configuration interaction spectroscopic constants for several small molecules. Even though highly correlated methods work best with larger basis sets, predicted spectroscopic constants are in good agreement with experiment; bond lengths and harmonic vibrational frequencies have average absolute errors no larger than 0.017 A and 1.6%, respectively, for all but the ANO basis. For the molecules considered, 6-31G** gives the smallest average errors, while the ANO basis set gives the largest. The use of variationally optimized basis sets and natural orbitals are also explored for improved benchmarking. Although optimized basis sets do not always improve predictions of molecular properties, taking a DZP-sized subset of the natu...

On the orbital dependence of compact, weight-selected configuration interaction and coupled-cluster wave functions

We recently reported that very compact coupled-cluster wave functions may be generated by selecting the most important configurations, by weight, from the full coupled-cluster wave function. Here, we consider how the choice of orbitals may affect these wave functions in the case of the symmetric dissociation of H2O. We employ unrestricted Hartree–Fock and complete-active-space self-consistent-field orbitals, as well as natural orbitals derived from a coupled-cluster singles and doubles wave function. For a given accuracy, some choices of orbitals can reduce the size of configuration interaction wave functions, but they have little effect on the weight-selected coupled-cluster wave functions.