The OPLS all-atom (AA) force field has been expanded to include squaramides and squaric acid. A complete set of stretching–bending, all-atom torsional and non-bonded parameters has been calculated. Starting from reported non-bonded parameters of amides, parameters were fitted to reproduce results from ab initio (MP2/6-) calculations on squaramides and squaric acid. Their application in the solution phase was tested by determining rotational barriers and cis/trans ratios of several squaramides in CHCl3 via Monte Carlo statistical mechanics simulations and comparing them with experimental ones determined by NMR experiments.