The SARS-CoV-2 virus initiates infection of human cells by recognizing the human angiotensin-converting enzyme 2 (ACE2) with the receptor binding domain (RBD) of the viral spike protein. Thus, the variant of concern (VOC) with mutations on RBD is of special interest. Here, we present a series of interaction analyses for the RBD-ACE2 complex of the wild-type (PDB ID: 6M0J) and those of B.1.1.7 (alpha), B.1.351 (beta) and P.1 (gamma) VOCs, based on the fragment molecular orbital (FMO) calculations. The results revealed that the RBD variants have a higher affinity for ACE2 than the wild type does.