POSCAR分数坐标与笛卡尔坐标变换

Basically, the fractional coordinates show the fraction of each unit cell vector that contributes to the position of that atom.

x_cart = A_x * frac_a + B_x * frac_b + C_x * frac_c
y_cart = A_y * frac_a + B_y * frac_b + C_y * frac_c
z_cart = A_z * frac_a + B_z * frac_b + C_z * frac_c

where A, B, and C are the unit cell vectors and A_x is the x-coordinate of unit cell vector A.

参考:

http://www.ccl.net/chemistry/resources/messages/2007/08/21.005-dir/

http://www.nyu.edu/classes/tuckerman/adv.chem/lectures/lecture_20/lecture_20.pdf