VASP计算介电常数

VASP加电场方法

EFIELD = X (units eV/A)：用于包含真空区域的Slab or Molecule 体系

注意：EEIELD实际上是电场力 (EFIELD is the magnitude of the electric force field for a test charge q (F = q E) and is measured in eV/Angstrom)，即EFIELD的单位是力F = qE， 所以对应电场的单位是E = F/q，因而1eV/A对应的场强1V/A，EFIELD正负号对应电场的正反反向.

此参数需要结合偶极修正相关参数来使用：

LDIPOL = .TRUE.
IDIPOL = direction of field
DIPOL = Rcenter

EFIELD_PEAD = Ex Ey Ez (eV/A)：用于Bulk体系

PEAD stands for Perturbation Expression After Discretization. The implementation of EFIELD_PEAD in VASP evaluates the influence of the external electric field on the electronic structure only. The contribution to forces acting on atoms is not impelemented. Therefore, the study of the influence of the external field on geometries is not possible.

静态介电常数两种计算方法

• Density Functional Perturbation Theory (DFPT，推荐此方法)

  LEPSILON = .TRUE.
  # the derivative of the cell-periodic part of the orbitals w.r.t. k is calculated using finite differences
  # as an alternative to solving a linear Sternheimer equation
  LPEAD    = .TRUE.
  # only includes local field effect on the Hartree part
  #LRPA     = .TRUE. 
  # to get the ionic contribution to the macroscopic dielectric tensor
  IBRION   = 8

  计算完成后从OUTCAR找到以下两部分并求和得到总的介电张量

  MACROSCOPIC STATIC DIELECTRIC TENSOR (including local field effects in DFT)
  
  MACROSCOPIC STATIC DIELECTRIC TENSOR IONIC CONTRIBUTION

• Linear Response to Finite Electric Fields

  LCALCEPS = .TRUE.
  # finite field requires very tight convergence,
  # as a default VASP will try for an even tighter convergence of the field-polarized groundstate: EDIFF/100
  EDIFF = 1E-8
  # to get the ionic contribution to the macroscopic dielectric tensor
  IBRION   = 6
  NFREE    = 2