Fast electronic structure methods

We developed a highly efficient and accurate electronic structure method for clusters of weakly interacting molecules. 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 polarisation or induction effects). The latter makes the combination of this method with existing implementations of any electronic structure theory (density functional, many-body perturbation, coupled-cluster, equation-of-motion coupled-cluster, configuration-interaction singles, and time-dependent density functional theories) extremely easy. The size dependence of the computational cost of the method is asymptotically linear for total energies and constant for excitation energies. 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. With this, we are planning to explore the vast field of water chemistry.