Investigation of electron-hole correlation using explicitly correlated configuration interaction method

► CI expansion is performed using explicitly correlated zeroth order reference wavefunction. ► Electron-hole cusp is described using Gaussian-type geminal function. ► The CI energy is the upper bound to the XCCI energy. ► Excited state energies are calculated from the explicitly correlated wavefunction. ► Excellent agreement between XCCI and R12-FCI energies.