Tensor Hypercontraction Equation-of-Motion Second-Order Approximate Coupled Cluster: Electronic Excitation Energies in O ( N 4 ) Time

The tensor hypercontraction (THC) formalism is applied to equation-of-motion second-order approximate coupled cluster singles and doubles (EOM-CC2). The resulting method, THC-EOM-CC2, is shown to scale as O ( N 4 ) , a reduction of one order from the formal O ( N 5 ) scaling of conventional EOM-CC2. Numerical tests for a variety of molecules show that errors of less than 0.02 eV are introduced into the excitation energies.