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洁如玉

铁虫 (小有名气)

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注册: 2012-10-07
性别: GG
专业: 凝聚态物性 II ：电子结构

[交流] 摘抄自百度文库的一篇关于计算partial charge的文章，很详细，值得初学者学习。

贺仪
2011-04-15 17:52 【转】用VASP进行Partial Charge分析实例
VASP Version : 4.6
在这篇文章中，我将首先介绍Partial Charge的概念，以及如何用VASP具体的计算Partial Charge。首先，所谓的Partial Charge是针对与Total Charge来说的，指的是某个能量范围、某个K点或者某个特定的态所对应的电荷密度。在文献中最常见的是价带顶部，导带底部，表面态或者局域态所对应的Partial Charge。通过分析这些态所对应的Partial Charge，可以得到体系的一些性质，比如局域态具体的是局域在哪个原子上等。我将通过具体的例子说明如何用VASP进行Partial Charge Analysis。
进行Partial Charge Analysis的第一步是进行自洽的计算，得到体系的电子结构。这一步的计算采用通常的INCAR和KPOINTS文件。在自洽计算结束后，我们需要保存WAVECAR文件。（通过在INCAR文件中设置LWAVE=TRUE实现）在这个例子中，假设我们需要计算一个硅纳米线的导带和价带的Partial Charge。硅纳米线的结构如下：
http://www.quantumchemistry.net/ ... 060331155154521.jpg

第二步是画出能带结构，以决定你需要画哪条能带的那个K点的态所对应的Partial Charge。关于具体如何用VASP画能带，请参见用VASP4.6计算晶体硅能带实例一文。我们得到硅纳米线的能带结构如下：

画能带时有些小技巧。你可以用一些支持列模块的编辑器，如UltraEdit，将OUTCAR里的各个K点所对应的本征值粘贴到Origin中。这一步完成后，在Origin中做一个矩阵转置，然后将K点坐标贴到第一列，并将其设为X坐标。如此画出来的基本上就是能带图了。在Origin中可以通过设置纵轴范围来更加清楚的区分费米能级附近的各条能带。如上的硅纳米线所对应的能带结构图如下：
http://www.quantumchemistry.net/ ... 060331155540648.jpg
决定画哪条能带，或者那些感兴趣的K点之后，有如下几种方法计算不同的Partial Charge。如果你希望计算价带顶端的Partial Charge，则需要首先通过能带结构图确定价带的能带标号。需要注意，进行Partial Charge分析必须要保留有自洽计算的WAVECAR才可以。
第一种Partial Charge分析的INCAR
ISTART =    1 job : 0-new 1-cont 2-samecut
ICHARG =    1 charge: 1-file 2-atom 10-const
LPARD=.TRUE.
IBAND= 20 21 22 23
KPUSE= 1 2 3 4
LSEPB=.TRUE.
LSEPK=.TRUE.
这样的INCAR给出的是指定能带，指定K点所对应的Partial Charge。分析导带、价带等的Partial Charge特性，通常采用的都是这种模式。
第二种Partial Charge分析的INCAR
ISTART =    1 job : 0-new 1-cont 2-samecut
ICHARG =    1 charge: 1-file 2-atom 10-const
LPARD=.TRUE.
EINT = -10.3 -5.1
LSEPB=.FALSE.
LSEPK=.FALSE.
这样的INCAR给出的是在[-10.3 -5.1]能量之间的Partial Charge。这种模式适合于分析某个能量区间内的波函数的性质。
第三种Partial Charge分析的INCAR
ISTART =    1 job : 0-new 1-cont 2-samecut
ICHARG =    1 charge: 1-file 2-atom 10-const
LPARD=.TRUE.
NBMOD=-3
EINT = -1
LSEPB=.FALSE.
LSEPK=.FALSE.
这样的INCAR给出的是从[Ef-1.0 Ef]能量之间的Partial Charge。这种模式最利于分析费米面附近的波函数的性质。
用第一种方法，我们可以得到硅纳米线价带顶部和导带底部的Partial Charge如下：
http://www.quantumchemistry.net/ ... 060331155753136.jpg

• LPARD: Evaluate partial (band and/or k-point) decomposed charge density. We want to stress again, that the
wavefunctions read from WAVECAR must be converged in a separate prior run. If only LPARD is set (and
none of the tags discussed below), the total charge density is evaluated from the wavefunctions and written
to CHGCAR.
• There are several ways how to specify for which bands the charge density is evaluated: In general the input
lines with IBAND, EINT and NBMOD control this respect of the routine:
• IBAND: Calculate the partial charge density for all bands speciﬁed in the array IBAND. If IBAND is speciﬁed
in the INCAR ﬁle and NBMOD is not given, NBMOD is set automatically to the size of the array. If IBAND is for
instance
IBAND= 20 21 22 23
the charge density will be calculated for bands 20 to 23.
• EINT: Speciﬁes the energy range of the bands that are used for the evaluation of the partial charge density.
Two real values should be given, if only one value is speciﬁed, the second one is set to ǫf . If EINT is given
and NBMOD is not speciﬁed, NBMOD is set automatically to -2.
• NBMOD: This integer variable can take the following values
> 0 Number of values in the array IBAND. If IBAND is speciﬁed, NBMOD is set automatically to the correct
value (in that case NBMOD should not be set manually in the INCAR ﬁle)
0 Take all bands to calculate the charge density, even unoccupied bands are taken into account.
-1 Calculate the total charge density as usual. This is the default value if nothing else is given.
-2 Calculate the partial charge density for electrons with there eigenvalues in the range speciﬁed by EINT.
-3 The same as before, but the energy range is given vs. the Fermi energy.
• KPUSE: KPUSE speciﬁes which k-points are used in the evaluation of the partial dos. KPUSE is an array of
integer values.
KPUSE= 1 2 3 4
means that the charge density is evaluated and summed for the ﬁrst four k-points. Be careful: VASP changes
the kpoint weights if KPUSE is speciﬁed.
• LSEPB: Speciﬁes whether the charge density is calculated for every band separately and written to a ﬁle
PARCHG.nb.⋆ (TRUE) or whether charge density is merged for all selected bands and write to the ﬁle
PARCHG.ALLB.⋆ or PARCHG. Default is FALSE.
• LSEPK: Speciﬁes whether the charge density of every k-point is write to the ﬁles PARCHG.⋆.nk (TRUE) or
whether it is merged (FALSE) to a single ﬁle. If the merged ﬁle is written, then the weight of each k-point is
determined from the KPOINTS ﬁle, otherwise the kpoints weights of one are chosen.

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