Extracting chemical information of free molecules from K-shell double core-hole spectroscopy

► Interatomic relaxation energy can be extracted from double core-hole spectroscopy. ► Interatomic relaxation is sensitive to the chemical environment. ► Interatomic relaxation decreases with the decrease in bond order. ► Interatomic relaxation decrease with the distance between the two core holes. ► F substation of H atoms induces the initial bonding shift and less affects the relaxation.