Symmetric Coupled Cluster Theory
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- Institutt for kjemi 
We introduce a novel coupled cluster formulation for the ground state, symmetric coupled cluster (SCC) theory, and an extension to excited states, molecular properties, and transition elements called symmetric equation of motion (SEOM) theory. The ground state cluster amplitudes and the expansion coefficients of the excited states are determined by stationarity of a Hermitian approximation of the Hamiltonian expectation value. We present an implementation of the ground state energy at the singles and doubles (SCCSD) truncation of the cluster operator. A series of calculations on molecular systems comprised of fewer than 50 electrons demonstrate that the SCCSD and CCSD energies are of similar accuracy. The method does not scale correctly with system size, and the errors introduced as a consequence limit its applicability to small systems.