Efficiency of a controlled use of Davidson's correction for the calculation of excitation energies:: vertical spectrum of trans-butadiene

As illustrated using the difficult problem of the trans-butadiene excitation spectrum, the use of the generalized Davidson's correction for the multireference CI description gives very reliable results when using the difference dedicated configuration interaction scheme, provided that the zero-order descriptions of the states involved are as good as possible and of similar quality. This balanced improvement is obtained by calculating average density matrices and using iteratively their mean natural orbitals. The results become quite stable when changing the size of the zero-order complete active spaces, and are in agreement with experimental values within a range of 0.1 eV.