For theoretical understanding of the microwave and radio frequency heating of water and for comparison to dielectric loss under microwave and radio-frequency irradiation, computational molecular modeling of water hydrogen-bonding aggregates, (H
2O)
3~6 is carried out on the basis of density-functional theory (DFT) using the B3LYP exchange-correlation functional and the 6–31G(d) basis set with Spartan 16 (Wave function, Inc. Irvine,CA) (DFT-based molecular modeling, DFT/MM). The DFT/MM-based IR/FIR spectrum of the water aggregates verifies that all aggregates absorb and dissipate of wide ranges of electromagnetic wave energy, i.e.,giving not only infrared (IR, 4000-500 cm
-1 ) but also far infrared (FIR, 500-0cm
-1) spectra. Absorption peaks inIR can recognize bond vibration, and absorption peaks in FIR recognize intermolecular vibration of aggregated molecules. On the basis of intermolecular vibration of absorption and dissipation spectra at FIR (0~500 cm
-1),which correspond to radio frequency of KHz and MHz, GHz(MW), THz(IR) region, and bond-stretching vibration spectra at IR (500~4000 cm
-1 ), we propose here that the radio and MW frequency heating may be explained as due to thermo-upconversion mechanism, i.e., final thermal dissipation at IR absorption at O-H stretching (3500~3000 cm
-1) caused by up-conversion of radio frequency energy absorbed by (H
2O)
3~6
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