Lattice energy is the energy needed to form crystals of a compound from the individual molecules. It is related to the stability of a compound in the solid state. In this study, systematic cluster method has been applied to obtain the lattice energy of urea. Using this method, the effect of solid state environment is included in a systematic way. Starting from the small clusters containing a few molecules, the largest cluster we studied contains 84 molecules. In order to improve the results from the cluster method using Gaussian 09 program, correction using the D3BJ program was included. The results show that, when compared to the experimental value, the lattice energies obtained were under-estimated for all the theoretical levels considered. Generally, application of the systematic cluster method shows decrease in calculated lattice energy as more molecules were included in the clusters and becomes closer to the experimental value of urea. Of all the levels considered, B3LYP/DEF2-TZVP with correctional terms provides the closest value to the one from the experiment.

We performed an investigation on urea interacting with hydroxyapatite (HA). The oxygen atoms on HA are either left alone or added with hydrogen to create hydroxyl to resemble the HA surface. Using B3LYP and 3 different basis sets, it was found that urea was able to interact positively with either hydroxyl or non-hydroxyl surface of HA. The Gaussian 09 and Multiwfn software were employed to conduct the calculations. The most favorable interaction has interaction energy of –1.36 eV, which was obtained with the 2 largest basis sets considered, on the pure hydroxyl surface. From the topology analysis on electron density and the non-covalent interaction analysis, it was found that the main attractions between urea and HA were due to the carbonyl oxygen and hydrogen of urea, and hydrogen, oxygen, and calcium on the HA surface. The bond length of newly bonded atoms ranges from 1.62 to 5.18 Å, whereas the energy gap has range between 0.46 to 1.14 eV. All the analysis performed in this study agreed with the results obtained in the formation of favorable interactions and complement previous experimental results that HA can bond with urea molecule. HIGHLIGHTS First known attempt to compare hydrogen-terminated and non-terminated surface of HA in urea adsorption The atoms involved in the adsorption were identified, and electronic structure of the possible combination was analysed and studied using topology and non-covalent interaction analysis Urea attached to both surfaces are stable, with negative interaction energy, confirming experiments’ results that urea can be adsorbed to HA GRAPHICAL ABSTRACT