本文为由小强撰写的《VASP实用教程》第40篇，全系列约60篇，将在近期陆续更新。
在前面的教程中和大家分享了差分电荷密度的计算方法，通过差分电荷密度，我们可以分析成键的过程或是结构弛豫前后电荷的转移，也可以分析吸附分子与衬底的电荷传输。
但是差分电荷密度只能定性的分析电荷传输情况，如果需要对电荷传输进行定性分析，则需要通过另外一种算法，即面积分差分电荷密度（plane-averaged charge density difference, PCDD）。
与差分电荷密度相同，面积分差分电荷密度也是通过处理CHGCAR文件得到的。下面我们说一下处理方法，以Z方向为例，步骤如下：
- chgdiff.pl file1/CHGCAR file2/CHGCAR，生成CHGCAR.diff文件，重命名为CHGCAR；
- vtotav.py CHGCAR Z，生成CHGCAR_Z文件。
注：
- chgdiff.pl为VTST·Tools网站提供，可自行下载。
- chgdiff.pl file1 file2 处理数据时，file2对应总的电荷，而file1对应的是需要减去的电荷；
- vtotav.py处理后的得到的CHGCAR_Z文件，可以直接导入origin作图，横坐标对应于Z轴的晶胞。使用vtotav.py要求系统已经安装ASE，安装方法参考上一篇教程。
vtotav.py的源代码：
#!/usr/bin/env python
"""
A script which averages a CHGCAR or LOCPOT file in one direction to make a 1D curve.
User must specify filename and direction on command line.
Depends on ase
"""
import os
import sys
import numpy as np
import math
import string
import datetime
import time
from ase.calculators.vasp import VaspChargeDensity
starttime = time.clock()
print ("Starting calculation at"),
print (time.strftime("%H:%M:%S on %a %d %b %Y"))
if len(sys.argv) != 3:
print ( "\n** ERROR: Must specify name of file and direction on command line.")
print ("eg. vtotav.py LOCPOT z.")
sys.exit(0)
if not os.path.isfile(sys.argv[1]):
print ("\n** ERROR: Input file %s was not found." % sys.argv[1])
sys.exit(0)
# Read information from command line
# First specify location of LOCPOT
LOCPOTfile = sys.argv[1].lstrip()
# Next the direction to make average in
# input should be x y z, or X Y Z. Default is Z.
allowed = "xyzXYZ"
direction = sys.argv[2].lstrip()
if allowed.find(direction) == -1 or len(direction)!=1 :
print ("** WARNING: The direction was input incorrectly.")
print ("** Setting to z-direction by default.")
if direction.islower():
direction = direction.upper()
filesuffix = "_%s" % direction
# Open geometry and density class objects
#—————————————–
vasp_charge = VaspChargeDensity(filename = LOCPOTfile)
potl = vasp_charge.chg[-1]
atoms = vasp_charge.atoms[-1]
del vasp_charge
# For LOCPOT files we multiply by the volume to get back to eV
if 'LOCPOT' in LOCPOTfile:
potl=potl*atoms.get_volume()
print ("\nReading file: %s" % LOCPOTfile)
print ("Performing average in %s direction" % direction)
# Read in lattice parameters and scale factor
#———————————————
cell = atoms.cell
# Find length of lattice vectors
#——————————–
latticelength = np.dot(cell, cell.T).diagonal()
latticelength = latticelength**0.5
# Read in potential data
#————————
ngridpts = np.array(potl.shape)
totgridpts = ngridpts.prod()
print ("Potential stored on a %dx%dx%d grid" % (ngridpts[0],ngridpts[1],ngridpts[2]))
print ("Total number of points is %d" % totgridpts)
print ("Reading potential data from file…"),
sys.stdout.flush()
print ("done.")
# Perform average
#—————–
if direction=="X":
idir = 0
a = 1
b = 2
elif direction=="Y":
a = 0
idir = 1
b = 2
else:
a = 0
b = 1
idir = 2
a = (idir+1)%3
b = (idir+2)%3
# At each point, sum over other two indices
average = np.zeros(ngridpts[idir],np.float)
for ipt in range(ngridpts[idir]):
if direction=="X":
average[ipt] = potl[ipt,:,:].sum()
elif direction=="Y":
average[ipt] = potl[:,ipt,:].sum()
else:
average[ipt] = potl[:,:,ipt].sum()
if 'LOCPOT' in LOCPOTfile:
# Scale by number of grid points in the plane.
# The resulting unit will be eV.
average /= ngridpts[a]*ngridpts[b]
else:
# Scale by size of area element in the plane,
# gives unit e/Ang. I.e. integrating the resulting
# CHG_dir file should give the total charge.
area = np.linalg.det([ (cell[a,a], cell[a,b] ),
(cell[b,a], cell[b,b])])
dA = area/(ngridpts[a]*ngridpts[b])
average *= dA
# Print out average
#——————-
averagefile = LOCPOTfile + filesuffix
print ("Writing averaged data to file %s…" % averagefile,)
sys.stdout.flush()
outputfile = open(averagefile,"w")
if 'LOCPOT' in LOCPOTfile:
outputfile.write("# Distance(Ang) Potential(eV)\n")
else:
outputfile.write("# Distance(Ang) Chg. density (e/Ang)\n")
xdiff = latticelength[idir]/float(ngridpts[idir]-1)
for i in range(ngridpts[idir]):
x = i*xdiff
outputfile.write("%15.8g %15.8g\n" % (x,average[i]))
outputfile.close()
print ("done.")
endtime = time.clock()
runtime = endtime-starttime
print ("\nEnd of calculation.")
print ("Program was running for %.2f seconds." % runtime)
