Van der Waals contributions in DFT calculations of cysteine-Au adsorption

Cysteine adsorption on Au surfaces, through its thiol group, is the primary mechanism for protein attachment on such surfaces. This bond is essential for developing protein-based bioelectronic devices because of the broad use of gold as an electrodes. When designing these devices, simulations almost always require molecular dynamics methods, but quantum mechanics methods are still needed to describe adsorption energy accurately. Density Functional Theory, with different van der Waals schemes, has become a handy tool for simulating molecule adsorption while retaining the chemical accuracy of QM methods. In this work, we have compared how the energetics of S-Au adsorption change with different DFT+vdW schemes. We found that PBE+DFT-D3 could be the preferred option over newer methods like SCAN-rvv10 as it yields similar results at a smaller computational cost. Also, DFT-D3 outperforms a specially modified version of DFT-D2 for S-Au adsorption, suggesting that the inclusion of the chemical environment in DFT-D3 is crucial for accurate results. Lastly, we compared cysteine to alkanethiols with different chain lengths and found that the chemical part of adsorption remains the same while most of the change comes from the physical adsorption and is directly related to the molecule size.