To verify the efficiency of scheme 2, we recalculated the above three cases but one atom density is modelled by a local mesh of 8∆x × 8∆x × 8∆x grids (∆x = 0.3 Å). Figure shows the comparison of calculation time after 300 steps between schemes 1 and 2. Blue bars indicate scheme 1 while red bars indicate scheme 2. In all cases, the calculation time in scheme 2 is shorter than in scheme 1. Moreover, the difference between calculation times expands as the number of atoms increases. Therefore, scheme 2 is more efficient than scheme 1 especially when the number of atoms increases.