INCAR文件中LREAL参数

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|  ADVICE TO THIS USER RUNNING 'VASP/VAMP'   (HEAR YOUR MASTER'S VOICE ...):  |
|                                                                             |
|      You have a (more or less) 'large supercell' and for larger cells       |
|      it might be more efficient to use real space projection operators      |
|      So try LREAL= Auto  in the INCAR   file.                               |
|      Mind: If you want to do a very accurate calculations keep the          |
|      reciprocal projection scheme          (i.e. LREAL=.FALSE.)             |
|                                                                             |
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INCAR文件中LREAL参数是什么意思，应该如何设置。
现将这个参数和不同选择项意思列于下，供参考。

LREAL: Default= .FALSE.
 

Determines whether the projection operators are evaluated in real-space or in reciprocal space:The non local part of the pseudopotential requires the evaluation of an expression  .The ``projected wavefunction character'' is defined as:

Currently VASP supports three different schemes to remove the high frequency components from the projectors. LREAL = .TRUE. is the simplest one. If LREAL = .TRUE. is selected,the real space projectors which have been generated by the pseudopotential generationcode are used. This requires no user interference.For LREAL = On the real space projectors are optimized by VASP using an algorithm proposed by King-Smith et al.[47]. For LREAL = Auto a new scheme [48] is used which isconsiderably better (resulting in more localized) projector functionsthan the King-Smith et al. method. To fine-tune the optimization procedure the flag ROPT can be usedif LREAL = Auto or LREAL = On is used.

We recommend to use the real-space projection scheme for systems containingmore than 20 atoms. We also recommend to use only LREAL = Auto  (for version VASP.4.4 and newer releases) and LREAL = On  (for all other versions).Version 4.4 also supports the old mode LREAL= O toallow calculations that are fully compatible to VASP.4.3 (and VASP.3.2).The best performance is generally achieved with LREAL= Auto, butif performance is not that important you can also use LREAL=.TRUE. whichgenerally requires less user interference. You can skip therest of the paragraph, if you use only LREAL=.TRUE..

For LREAL = O and LREAL = Athe projection operators are optimized by VASPon the fly (i.e. on startup). Several flags influence the optimization

• ENCUT (i.e. the energy cutoff), components beyond the energy cutoff are 'removed' from the projection operators.

• PREC tag specifies how precise the real space projectorsshould be, and sets the variables ROPT  accordingly to the followingvalues:

  For LREAL = On

  PREC = Low	700 points in the real space sphere (ROPT=0.67)
  PREC = Med	1000 points in the real space sphere (ROPT=1.0)
  PREC = High	1500 points in the real space sphere (ROPT=1.5)

  
  
  

  For LREAL = Auto

  PREC = Low	accuracy (ROPT=0.01)
  PREC = Med	accuracy 2 (ROPT=0.002)
  PREC = High	accuracy 2 (ROPT=2E-4)

  
  
  

  These defaults can be superseded by the line

  ROPT = one_number_for_each_species in the INCAR file. For instance ROPT = 0.7 1.5 will set the number of real space points within the cutoff sphere for the first speciesto approximately 700, and that for the second species to 1500.In VASP.4.4 alternatively the ``precision'' of the operators can be specifiedwriting i.e.ROPT = 1E-3 1E-3 In that case the real space operators will be optimized foran accuracy of approximately 1meV/atom (). The ``precision'' mode works both for LREAL=On and LREAL=Auto (but to maintaincompatibility with older VASP versions it  is only selected if LREAL = Auto is specified in the INCAR file).The precision mode is generally switched on if the value for ROPTis smaller than 0.1. The ``precision'' mode and the conventional mode can be intermixed,i.e. it is possible to specifyROPT = 0.7 1E-3 in that case the number of real space points within the cutoff sphere for the first specieswill be approximately 700, whereas the real space projector functions for the secondspecies are optimized for an accuracy of approximately 1 meV.We recommend to use the ``precision'' mode  with a target accuracy of around eV/atom if your version supports this.

  If you use the mode in which the number of grid points in the real space projection sphere is specified, you have to selectROPT carefully, especially if a hard species is mixed with a softspecies. In that case the following lines in the OUTCAR file mustbe checked (here is the output for LREAL = On, but that one forLREAL = Auto is quite similar )

  Optimization of the real space projectors maximal supplied Q-value = 12.85 optimization between [QCUT,QGAM] = [ 4.75, 9.51] = [ 6.33, 25.32] Ry Optimized for a Real-space Cutoff 2.30 Angstroem l X(QCUT) X(cont) X(QGAM) max X(q) W(q)/X(q) e(spline) 0 9.518 9.484 -.004 18.582 .11E-03 .16E-06 0 -2.149 -2.145 .001 3.059 .17E-03 .25E-06 1 8.957 8.942 .003 9.950 .14E-03 .34E-06 1 1.870 1.870 .001 1.837 .95E-03 .51E-06 2 3.874 3.866 .000 4.764 .15E-03 .68E-07 The meaning of QCUT and QGAM is explained in Sec. 11.5.6. The most important information is given in thecolumn W(q)/X(q) (respectively the column  W(low)/X(q) forLREAL = Auto).The values in these columns must  be as small as possible.If these values are too large, increase the ROPT tag from the default value.As a rule of thumb the maximum allowed value in this column is for PREC = Med.(For  PREC = Low errors might be around and for PREC = High errorsshould be smaller than ). If  W(q)/X(q) is larger than theerrors introduced by the real space projections can be substantial.In this case ROPTmust be specified in the INCAR file to avoidincorrect results. If the new precision mode is used in VASP.4.4 (ROPT 0.1) thecode automatically selects the real-space cutoff  so that therequired precision is reached.

A few comments for non-experts and experts:Real space optimization (LREAL = .TRUE., LREAL = On or  LREAL = Auto)always results in a small (not necessarily negligible)error (the error is usually a constant energy shift for each atom). If you are interested in energy differences of a fewmeV use only calculations with the same setup (i.e. same ENCUT,PREC, LREAL and ROPT setting) forall calculations.For example, if you want to calculatesurface energies recalculate the bulk groundstate energy withexactly the same setting you are going to use for the surface. Another possibility is to relax the surface with real space projection, and to do one final total energy calculation with LREAL = .FALSE.  to get exact energies. Anyway, for PREC = Med, the errors introduced by the real spaceprojection are usually of the same order magnitude as those introduced by the wrap around errors.For PREC = High errors are usually less than meV. PREC = Lowshould be used only for high speed MD's,if computer resources are really a problem.