XCC2-a new coupled cluster model for the second-order polarization propagator

A new coupled cluster model of the polarization propagator, denoted as XCC2, is presented. The XCC2 approach employs time-independent coupled cluster theory of polarization propagators of Moszynski et al. [Collect. Czech. Chem. Commun., 2005, 70, 1109] and excitation operators from the time-dependent (TD) CC2 method. The performance of XCC2 was investigated by calculating static and dynamic dipole polarizabilities for a test set of over 20 molecules and comparing them with TD-CCSD results. The quality of XCC2 dispersion coefficients for several noncovalent molecular complexes was also tested against the benchmark values. This numerical study reveals that the average percent error of XCC2 is significantly reduced in comparison to the TD-CC2 method (4-fold reduction for the mean polarizabilities and 2-fold reduction for anisotropic polarizabilities is observed). Since the computational requirements of both XCC2 and TD-CC2 methods are virtually the same, the new XCC2 method can be viewed as a practical alternative for TD-CC2 for property calculations, giving the second-order polarization propagators of near-CCSD quality in many cases, but retaining at the same time the lower computational cost of the TD-CC2 model.