Comparison of multireference Moller-Plesset theory and time-dependent methods for the calculation of vertical excitation energies of molecules

The vertical singlet-singlet excitation energies for a benchmark- set of 14 medium and large molecules have been investigated with three quantum chemical methods. Calculations for electronic states with very different character in organic and inorganic systems are used to assess the accuracy and applicability of a simplified multireference Moller-Plesset (MR-MP2) approach, time-dependent density functional theory (TDDFT), and an approximate coupled cluster method with single and double excitations (CC2). In the pure ab initio approaches the resolution of the identity (RI) method for the calculation of the two-electron integrals is used to improve computational efficiency. It is shown that independently of the complexity of the electronic states involved, only the MR-MP2 method yields high accuracy (mean absolute deviation of 0.14 eV for 22 states). This finding is of particular importance because our scheme avoids computationally demanding orbital optimization steps and employs very compact reference wave functions. The TDDFT results are significantly poorer (mean absolute deviation of 0.26 eV), and systematic deviations for some pi --> pi* states, Rydberg states, and systems with unusual electronic structure are obtained. It is concluded that TDDFT has a potential for exploratory investigations or for very large molecules due to its computational efficiency. The CC2 method shows a tendency to overestimate excitation energies and is also limited to systems where the ground state is well described by a single determinant.