Atomic Natural Orbital Basis Research Articles

Core–core and core–valence correlation

The effect of 1s core correlation on properties and energy separations has been analyzed using full configuration-interaction (FCI) calculations. The Be 1S–1P, the C 3P–5S, and CH+ 1Σ+–1Π separations, and CH+ spectroscopic constants, dipole moment and 1Σ+–1Π transition dipole moment have been studied. The results of the FCI calculations are compared to those obtained using approximate methods. In addition, the generation of atomic natural orbital (ANO) basis sets, as a method for contracting a primitive basis set for both valence and core correlation, is discussed. When both core–core and core–valence correlation are included in the calculation, no suitable truncated CI approach consistently reproduces the FCI, and contraction of the basis set is very difficult. If the (nearly constant) core–core correlation is eliminated, and only the core–valence correlation is included, CASSCF/MRCI approaches reproduce the FCI results and basis set contraction is significantly easier.

Theoretical studies of the potential surface for the F+H2→HF+H reaction

The F+H2→HF+H potential energy hypersurface has been studied in the saddle-point and entrance channel regions. Using a large [5s 5p 3d 2f 1g/4s 3p 2d] atomic natural orbital basis set, we obtain a classical barrier height of 1.86 kcal/mol at the CASSCF/multireference CI level (MRCI) after correcting for basis set superposition error and including a Davidson correction (+Q) for higher excitations. Based upon an analysis of the computed results, the true classical barrier is estimated to be about 1.4 kcal/mol. We also compute the location of the bottleneck on the lowest vibrationally adiabatic potential curve, and determine the translational energy threshold from a one-dimensional tunneling calculation. Using the difference between the calculated and experimental threshold to adjust the classical barrier height on the computed surface yields a classical barrier in the range of 1.0–1.5 kcal/mol. Combining the results of our direct estimates of the classical barrier height with the empirical values obtained from our approximate calculations of the dynamical threshold, we predict that the true classical barrier height is 1.4±0.4 kcal/mol. Arguments are presented in favor of including the relatively large (≈1 kcal/mol)+Q correction obtained when nine electrons are correlated at the CASSCF/MRCI level.

Theoretical study of the N+2 Meinel system

The potential curves for the X 2Σ+g and A 2Πu states of N+2 are determined using generally contracted atomic natural orbital Gaussian basis sets and large CASSCF/multireference CI wave functions. The dependence of the X 2Σ+g– A 2Πu electronic transition moment on internuclear distance is accurately determined, and is shown to differ significantly from previous determinations. However, the theoretically determined lifetimes agree with the experimental time-of-flight data of Peterson and Moseley and of Holland and Maier within the stated experimental error bars.