A resonance theory consistent with Mulliken-population concept

► An analysis of molecular orbital wave function from the viewpoint of resonance theory is proposed. ► The present method is very easy-to-use and provides precise view of the molecular orbitals through a projection based on the second quantization. ► The present method is applicable to a conjugated-electron system and enables us to calculate the weights of resonance structures. ► Different from analogues, the present computed weight is consistent with Mulliken population as well as bond order analysis.