A method, which includes: extracting one or more calculation-target water molecules that influence at least interaction energy between the target molecule and some of the drug candidate molecules, but not the interaction energy between the target molecule and all of the drug candidate molecules; and calculating each interaction energy between the target molecule and each of the drug candidate molecules with only considering the one or more calculation-target water molecules as water molecules treated positively in a calculation of each interaction energy, and with considering the one or more calculation-target water molecules only in a calculation of interaction energy which the one or more calculation-target water molecules influence, wherein the method is a method for calculating interaction energy between the target molecule and the drug candidate molecules using a calculator.