Mixed ab initio and semiempirical study of hydrogen-terminated finite germanium nanowires

We present a mixed ab initio and semiempirical method for the cohesive energy and electronic gap calculations of hydrogen passivated tetrahedral and clathrate germanium nanowires (1850 atoms) with acceptable accuracy, comparable to density functional theory results, and with a significantly lower computational cost. First, we find that the PM6 semiempirical method produce the most accurate geometries when compared with the DFT results; whereas other semiempirical methods such as AM1, PM3 and PM7 clearly underestimate (or overestimate). Second, we implement the DFT@PM6 mixed scheme for cohesive/binding energy and electronic gap calculations which shows promising results compared with reference values of DFT. However, the bulk energy gap and binding energy values from the quantum confinement fitting procedure slightly underestimate the results which can be easily overcome using suitable functional and basis set/ECP. Also, a comparison with previous work clearly shows that the calculated electronic gap for bulk germanium is extremely sensitive to the choice of framework. Further development in this research work is progressing.