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

摘要

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 em- phasized 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.