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In this work, we revisited the idea of using the coupled-cluster ground stateformalism to target excited states. Our main focus was targeting doubly excitedstates and double core hole states. Typical equation-of-motion (EOM) approachesfor obtaining these states struggle without higher-order excitations thandoubles. We showed that by using a non-aufbau determinant optimized via themaximum overlap method the CC ground state solver can target higher energystates. Furthermore, just with singles and doubles (i.e., CCSD), wedemonstrated that the accuracy of $\Delta$CCSD and $\Delta$CCSD(T) farsurpasses that of EOM-CCSD for doubly excited states. The accuracy of$\Delta$CCSD(T) is nearly exact for doubly excited states considered in thiswork. For double core hole states, we used an improved ansatz for greaternumerical stability by freezing core hole orbitals. The improved methods, corevalence separation (CVS)-$\Delta$CCSD and CVS-$\Delta$CCSD(T), were applied tothe calculation of the double ionization potential of small molecules. Evenwithout relativistic corrections, we observed qualitatively accurate resultswith CVS-$\Delta$CCSD and CVS-$\Delta$CCSD(T). Remaining challenges in$\Delta$CC include the description of open-shell singlet excited states withthe single-reference CC ground state formalism as well as excited states withgenuine multi-reference character. The tools and intuition developed in thiswork may serve as a stepping stone towards directly targeting arbitrary excitedstates using ground state CC methods.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Excited States via Coupled Cluster Theory without Equation-of-Motion  Methods: Seeking Higher Roots with Application to Doubly Excited States and  Double Core Hole States