Variation in spin contamination and diradical character with distance between a singlet biradical molecule and surface

As diradical molecules are used to perform various unique functions, they are expected to be applied as molecular devices. In addition, theoretical calculations based on density functional theory (DFT) are critical tools in the field of computational material science. In the DFT calculations of a diradical molecule, investigation of the spin contamination error and estimation of the diradical character, which is a feature of diradical molecules, are necessary. However, little is known regarding the variation in the spin contamination error and diradical character with the distance from a surface. These data are essential in the fields of physical chemistry and surface science to enable the study of the effects of surface interactions on strongly correlated electrons. Herein, the variations were clarified by performing approximate spin-projection DFT/plane-wave (AP-DFT/plane-wave) calculations using three reaction model systems: (1) diradical adsorption state → non-diradical molecule, (2) non-diradical adsorption state → diradical molecule, and (3) diradical adsorption state → diradical molecule. Variations in the diradical character were monotonic, whereas the spin contamination error exhibited extreme values and inflections. This was because the interaction with the surface exhibited two opposing effects: (1) the diradical character of the adsorbate increased, which increased the spin contamination error, and (2) the energy difference between the open singlet and high-spin states decreased, which decreased the spin contamination error. When a diradical/non-diradical transformed owing to surface adsorption, the effect of the spin contamination error on the adsorption energy became large. Hence, spin contamination should be analysed when the considered adsorption system features diradical/non-diradical transformations. In addition, the diradical character was affected by a surface even when dispersion forces were the major adsorption stabilisation factor. This indicated that the optimisation of the surface interactions was critical in the immobilisation of diradical molecules.