Basis set superposition error (BSSE) correction with counterpoise (CP) procedure under the environmental electrostatic potential is newly introduced to interfragment interaction energy (IFIE), which is important for interaction analysis in the fragment molecular orbital method. The CP correction for IFIE is applied to a stacked dimer of base pair and a protein-ligand complex of estrogen receptor and 17β-estradiol with scaled third-order Møller-Plesset perturbation theory. The BSSEs amount to about quarter of IFIE for hydrogen-bonding and electrostatic interactions and half or even more for dispersion interactions. Estimation of IFIE with the CP correction is therefore preferred for the quantitative discussion.
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