The physics-based potentials of side-chain side-chain interactions corresponding to pairs, composed of charged and polar, polar and polar, charged and hydrophobic, and hydrophobic and hydrophobic side chains have been determined. A total of 144 four-dimensional potentials of mean force (PMFs) of all possible pairs of molecules modeling these pairs were determined. by umbrella-sampling molecular dynamics simulations in explicit water as functions of distance and orientation, :and the analytical expressions were then fitted: to the PMFs. Depending On the type of interacting sites, the analytical approximation to the PMF is a sum of terms corresponding to van der Waals interactions and cavity-creation involving the nonpolar sections of the side chains and van der Waals, cavity-creation, and electrostatic (charge -dipole or dipole-dipole) interaction energies and polarization energies involving the charged Or polar sections of the side chains. The model used in this work reproduces all features of the interacting pairs. The UNited RESidue force field with the new side-chain side-chain interaction potentials was preliminarily tested with the N-terminal part of the B-domain of staphylocciecal protein A (PDBL 1BDD; a three-alpha-helix bundle) and UPF0291 protein YnzC from Bacillus subtilis (PDB: 2HEP; an alpha-helical hairpin).