Davidson corrected multireference calculations

Using DMBE IV potential, Meijer and coworkers carried out wavepacket calculations of the initial state selected total cross sections for the H + O2, including partial waves up to / = 35. All of the projections of J onto the intermolecular axis have been incorporated in the calculations. They found that the calculated cross sections are lower than the experiment, which indicated the deficiencies in the DMBE IV potentials. In 2005, Xu et al. constructed a new potential (XXZLG PES) for this reaction at the internally contracted multireference configuration interaction plus the Davidson correction level with the augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis set. It has been shown that there is remarkable improvement over the previous DMBE IV potential. Based upon this new potential and using the recent developed RGB quantum wave packet method. Sun et al. calculated state-to-state DCS and ICS of the H + O2 reaction up to 1.5 eV. 

Regardless of whether a single- or multireference wavefunction is used for a CISD calculation, the effects of higher excitations (e.g., triples and quadruples) are obviously not included. Quadruple excitations are particularly important because it is their absence from CISD wavefunctions for closed-shell molecules that results in these wavefunctions not being size consistent. Methods for estimating the effect of quadruple excitations have been developed, both by Davidson and by Pople. The Davidson corrections for quadruples can easily be computed by hand, but they are also automatically calculated as part of the CISD modules in the MOLPRO and MOLCAS programs. 

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