Quantum Chemistry of Excited State: Tamm-Dankoff Approximation with Correlated Wave Functions

A simple derivation of the general equations of the Tamm-Dankoff approximation (TDA) is presented using the equation-of-motion technique to describe electronic excitations in molecules.It is emphasized that the performance of this method strongly depends on the accuracy of the reference (ground) state.Though the Hartree-Fock ground state is commonly applied, the 'all-single CI' (CIS) method based on it is not too reliable.On the other hand, if the ground state is described by sophisticated wave functions like CISD or a coupled cluster ansatz, the TDA equations become quite complicated and may even turn inconsistent.We advocate the use of geminal type ground state wave functions, which, if the strong orthogonality condition is utilized, provide an efficient starting point, being not only highly correlated but also very transparent.Fully consistent TDA equations are derived for strongly orthogonal geminals, which can be of great help in the interpretation of molecular spectra in terms of local contributions and chromophores.CROATICA CHEMICA ACTA CCACAA 71 (3) 489¿499 (1998)