Roughly speaking, the formation energy first decreases linearly with the increase of Li when x < 2, then the decreasing speed decreases when 2 < x < 3, and finally the curve approaches a plateau when We next turn to the detailed local shape of the formation energy curve. As shown in the enlarged part of the D-c curve with x < 0.3, the formation energy suddenly increases by a small value at x ~ 0.06 (see the arrows in the illustration in Figure ). Though less obvious, this phenomenon also exists in L-c, L-a and D-a curves. This sudden increase corresponds to the change from the stable solid solution of Li in Si to a-LixSi phase, and the corresponding snapshot can be seen in the initial stage of L-c reported in the section below. Therefore, the simulations broadly capture the lithiation/delithiation processes of Si anode, and we obtain a series of relaxed meta-stable amorphous structures.