DFT simulation of vibrational properties of adenine adsorbed on gold surface: The effect of periodic boundary conditions

We performed simulations of the vibrational properties of the adenine adsorbed on Au(1 0 0) gold surface by using three different geometric models. For each geometric model we used two approximations to build partial the Hessian matrix: in the first one the gold atoms were allowed to vibrate while in the second one they are pinned to their relaxed positions. The influence of the vibrations of the first layers in the gold surface upon the vibrational bands of the adenine as well as the influence of a point-like defect in the surface is discussed in detail. The comparison with experimental results indicates that two maxima of the SERS signals are present (just above 300 and 600 cm−1) while the values obtained from our calculations are 327 cm−1 and 625 cm−1. For the frequencies ranging between 0 and 700 cm−1 we note shifts of up to 40 cm−1 caused by the use of different computational models. For the frequencies larger than 700 cm−1 there are practically no differences between the models used by us. The presence of a point-like defect on the surface leads to important modification of the vibrational properties of adenine in the low frequency regime. While in the case of VbDOS projected over the gold atoms the presence of the defect leads to minimal changes of the vibrational density of states, the influence of the defect of the adenine’s atoms is more important. This is clearly visible by comparing the VbDOS at low frequencies.