Intermolecular potential functions have been developed for urea. Ab initio 6–31G(d) calculations were performed on urea‐water complexes to obtain interaction energies and hydrogen‐bond lengths that were used in developing the partial charges for the OPLS model. An important test was made by computing through Monte Carlo simulations the difference in chloroform/water partition coefficients for urea and acetamide, Δ log P. The accord between the computed result from statistical perturbation theory for TIP4P water and OPLS chloroform (2.0 ± 0.1 ) and the experimental value (1.9) is excellent. The computed absolute free energy of hydration of acetamide (‐9.5 ± 0.4 kcal/mol) also matches the experimental data at 25 °C (‐9.68 kcal/mol). These facts provide confidence in the computed value of −13.6 ± 0.4 kcal/mol for the absolute free energy of hydration of urea, an experimentally unavailable result. The water structure around urea and acetamide was also characterized; the average numbers of solute‐water hydrogen bonds are 5 for urea and 3–4 for acetamide.
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