Explicitly-correlated double ionization potentials and double electron attachment equation-of-motion coupled cluster methods

Highlights: • The DIP-EOM-CCSD(F12) and DEA-EOM-CCSD(F12) methods are implemented. • Highly accurate double ionization potentials and double electron affinities are obtained for the set of molecules. • Accurate results can be obtained already at double-zeta level. Double ionization and double electron attachment equation-of-motion methods, based on linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] are formulated and implemented. An extension of double electron attachment operator is introduced for proper account of short-range correlation effects in states with two additional electrons. Numerical tests for set of doubly ionized and doubly electron attached states of several molecules have shown a good agreement between obtained explicitly-correlated results and the corresponding complete basis set limit values already at double-$\zeta$ level.