PCI-X, A PARAMETRIZED CORRELATION METHOD CONTAINING A SINGLE ADJUSTABLE PARAMETER-X

A new method intended for the calculation of bond strengths is suggested and tested on a large variety of systems. The method is based on the simple fact that in a balanced treatment about the same percentage X of the correlation effects is obtained for every system. The total energy is then obtained by a simple extrapolation to 100 percent. Using a double zeta plus polarization basis set in a coupled cluster type calculation it is found that the percentage X is about 80. Small changes of the basis sets or methods do not change this percentage significantly. Since the method is parametrized and the underlying method is some type of configuration interaction the method is termed PCI-X. For a selection of 13 representative simple first-row molecules the present version of the PCI-80 method gives an average absolute deviation from experiment of 1.8 kcal/mol compared to 10.3 kcal/mol for the unparametrized calculations. For some of these bond strengths a Hartree-Fock limit correction is required. For transition metal complexes, for which the method is primarily aimed, the improvement can be quite dramatic compared to a normal standard treatment. The method is tested on essentially all small second-row transition metal systems studied experimentally, for 38 systems, and the average absolute deviation from these sometimes rather uncertain experiments is 5.1 kcal/mol (0.22 eV).