Fast electron correlation methods for molecular clusters in the ground and excited states

An efficient and accurate electronic structure method for clusters of weakly interacting molecules has been proposed, on the basis of the pair-interaction method of Kitaura et al., and combined with density functional, many-body perturbation, coupled-cluster, equation-of-motion coupled-cluster, configuration-interaction singles, and time-dependent density functional theories. The method retains the one- and two-body (and, if necessary, also three-body) Coulomb, exchange, and correlation energies exactly and higher-order Coulomb energies in the leading order of multipole expansion (hence the dipole polarization or induction effects). The latter makes the combination of this method with existing implementations of any electronic structure theory extremely easy. It typically recovers the total energies within 0.001%, binding energies within a few kilocalories per mole, and excitation energies within a few hundredths of an electron volt of the conventional implementations. The size dependence of the computational cost of the method is asymptotically linear for total energies and constant for excitation energies. The method has been applied to the total energies of water clusters, to the total energies of zwitterionic and neutral glycine–water clusters, and to the excitation energies of formaldehyde–water clusters. The largest calculation was performed at an equation-of-motion coupled-cluster singles and doubles level for a formaldehyde–(H2O)81 cluster containing 247 atoms that predicted the solvatochromic shift of 1360 cm−1 in the lowest transition energy of formaldehyde in water.