VASP(计算前的)验证.doc

(计算前的)验证
一、检验赝势的好坏:
(一)方法:对单个原子进行计算;
(二)要求:1、对称性和自旋极化均采用默认值;
2、ENCUT要足够大;
3、原胞的大小要足够大,一般设置为15 Å足矣,对某些元素还可以取得更小一些。
(三)以计算单个Fe原子为例:
1、INCAR文件:
SYSTEM = Fe atom
ENCUT = eV
NELMDL = 5 ! make five delays till charge mixing,详细意义见注释一
ISMEAR = 0
SIGMA=
2、POSCAR文件:
atom




1
Direct
0 0 0
3、KPOINTS文件:(详细解释见注释二。)
Automatic
0
Gamma
1 1 1
0 0 0
4、POTCAR文件:(略)
注释一:关键词“NELMDL”:
A)此关键词的用途:指定计算开始时电子非自洽迭代的步数(即
NELMDL gives the number of non-selfconsistent steps at the beginning),目的是make calculations faster。“非自洽”指的是保持charge density不变,由于Charge density is used to set up the Hamiltonian, 所以“非自洽”也指保持初始的哈密顿量不变。
B)默认值(default value):
NELMDL = -5 (当ISTART=0, INIWAV=1, and IALGO=8时)
   NELMDL = -12 (当 ISTART=0, INIWAV=1, and IALGO=48时)
   NELMDL = 0 (其他情况下)
NELMDL might be positive or negative.
A positive number means that a delay is applied after each ionic movement -- in general not a convenient option. (在每次核运动之后)
A negative value results in a delay only for the start-configuration. (只在第一步核运动之前)
C)关键词“NELMDL”为什么可以减少计算所需的时间?
Charge density is used to set up the Hamiltonian, then the wavefunctions are optimized iteratively so that they get closer to the exact wavefunctions of this Hamiltonian. From the optimized wavefunctions a new charge density is calculated, which is then mixed with the old input-cha