Averaged Coupled-pair Functional Research Articles

Theoretical study of the nitrogen-atom hyperfine coupling constant. II

The N hyperfine coupling constant has been computed using multireference configuration interaction (MRCI) and averaged coupled pair functional (ACPF) treatments in very large one-particle basis sets. Unlike previous calculations, no selection of configurations has been performed. The calculations again illustrate the difficulty of computing the N hyperfine coupling constant accurately. The best MRCI result of 10.7 MHz is larger than the accurate experimental value of 10.4509 MHz, while the best ACPF result (10.3 MHz) is smaller than experiment. The difference between this work and previous calculations is discussed.

The calculation of the dipole moments of NiH, TiO, and FeO

We explore several computational strategies for computing the dipole moment of theX2Δ state of NiH, theX3Δ state of TiO, and theX5Δ state of FeO. The averaged coupled-pair functional (ACPF) method gives consistently the best agreement with experiment, but can become intractable, as rather large zeroth-order reference spaces can be required. At the ACPF level, unlike the multireference configuration-interaction (MRCI) level, the dipole moments determined as an expectation value and by finite-field methods are similar, and are insensitive to natural orbital iteration. Our best theoretical results for NiH are in excellent agreement with experiment, whereas our best dipole moments for TiO and FeO are both about 10%–15% larger than the recently measured values.

The effect of higher than double excitations on the F+H2→FH+H barrier

The classical barrier height and saddle point geometry are computed using the averaged coupled-pair functional (ACPF) method, correlating both seven and nine electrons. The size-consistent ACPF method indicates that 2s correlation substantially reduces the barrier. The excellent agreement between the ACPF and MRCI+Q results at both the seven and nine electron level provides additional support for the +Q correction. Thus the ACPF treatment supports a low (1.65 kcal/mol) barrier. The seven-electron CISDTQ results are shown to be consistent with the MRCI+Q and ACPF results.

The averaged coupled-pair functional (ACPF): A size-extensive modification of MR CI(SD)

To account for unlinked-cluster effects at the multireference level, we propose a generalization (MR ACPF) of the “coupled-pair functional” (CPF). Comparison with full CI results for BeH 2, H 2O, CH 2, O 2 and the five lowest-lying 2A 1 states of CH 2 +, shows the superiority of MR ACPF over MR CI(SD) and MR LCCM. Large-scale calculations for F 2 (up to a [7s5p3d2flg] CGTO basis set and 6 references) show better convergence to the experimental D e values on basis set and reference space extension than does MR CI(SD).