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VdW-DF functional of Langreth and Lundqvist et al.
The vdW-DF proposed by Dion et al. is a non-local correlation functional that approximately accounts for dispersion interactions. In VASP the method is implemented using the algorithm of Roman-Perez and Soler which transforms the double real space integral to reciprocal space and reduces the computational effort. Several proposed versions of the method can be used: the original vdW-DF, the "opt" functionals (optPBE-vdW, optB88-vdW, and optB86b-vdW) where the exchange functionals were optimised for the correlation part, and the vdW-DF2 of Langreth and Lundqvist groups.
This method is available since the 184.108.40.206May2011 version of VASP for the calculation of total energies and forces. The stress calculation for the cell optimisation (ISIF=3}) is available since the VASP 220.127.116.11Nov2011 version for spin unpolarised systems and VASP 5.3.1 for spin polarised systems.
N.B.: This feature has been implemented by J. Klimeš. If you make use of the vdW-DF functionals presented in this section, we ask you to cite reference . Please also cite the original vdW-DF paper of Dion et al. and the paper of Roman-Perez and Soler. In addtion, cite the paper of Lee et al. if you use the vdW-DF2 functional, the paper of Klimeš et al. if you use the optB88-vdW or optPBE-vdW functionals, and any other appropriate references, such as reference.
The method is invoked by setting LUSE_VDW=.TRUE.. Moreover, the PBE correlation correction needs to be removed since only LDA correlation is used in the functionals. This is done by setting AGGAC=0.0000.
The two tags above need to be used for all of the following functionals, i.e.:
The flag LASPH is strongly recommended for vdW-DFT, since often vdW-DFT yields less spherical densities than standard DFT. To get reasonably accurate contributions from the spheres around the atoms, it is recommended to set this flag.
The GGA tag is further used to choose the appropriate exchange functional.
- The original vdW-DF of Dion et al uses revPBE, therefore the vdW-DF can be chosen by setting
- More accurate exchange functionals (optPBE, optB88, and optB86b) for the vdW correlation functional have been proposed in references  and .
- For optPBE-vdW set:
- For optB88-vdW set:
GGA = BO PARAM1 = 0.1833333333 PARAM2 = 0.2200000000 LUSE_VDW = .TRUE. AGGAC = 0.0000 LASPH = .TRUE.
- And for optB86b-vdW:
- In the vdW-DF2 functional the rPW86 exchange functional is used (GGA=ML). Moreover, the vdW functional needs to be changed to the vdW2 correlation which requires only a change of a parameter (J. Klimeš) Zab_vdW=-1.8867.
- Therefore to use vdW-DF2, set:
- The rev-vdW-DF2 functional of Hamada, also known as vdW-DF2-B86R, can be selected by setting :
GGA = MK LUSE_VDW = .TRUE. PARAM1 = 0.1234 PARAM2 = 0.711357 Zab_vdW = -1.8867 AGGAC = 0.0000 LASPH = .TRUE.
- To select the SCAN + rVV10 functional of Peng et al.  set:
METAGGA = SCAN LUSE_VDW = .TRUE. BPARAM = 6.3 # default but can be overwritten by this tag CPARAM = 0.0093 # default but can be overwritten by this tag LASPH = .TRUE.
- Presently, it is not possible to combine SCAN with vdW-DFT functionals other than rVV10.
- NOTE: As of vasp.6.2 (and prior version) the stress tensor seems to be broken for rVV10. It is correct for other vdW-DF though.
- The method needs a precalculated kernel which is distributed via the VASP download portal (VASP -> src -> vdw_kernel.bindat) and on the ftp server (vasp5/src/vdw_kernel.bindat). If VASP does not find this file, the kernel will be calculated. This, however, is a rather demanding calculation. The kernel needs to be either copied to the VASP run directory for each calculation or can be stored in a central location and read from there. The location needs to be set in routine PHI_GENERATE. This does not work on some clusters and the kernel needs to be copied into the run directory in such cases. The distributed file uses little endian convention and won't be read on big endian machines. The big endian version of the file is available from the VASP team.
- There are no special POTCAR files for the vdW-DF functionals and the PBE or LDA POTCAR files can be used. Currently the evaluation of the vdW energy term is not done fully within the PAW method but the sum of the pseudo-valence density and partial core density is used. This approximation works rather well, as is discussed in , and the accuracy generally increases when the number of valence electrons is increased or when harder PAW datasets are used. For example, for adsorption it is recommended to compare the adsorption energy obtained with standard PAW datasets and more-electron POTCAR files for both PBE calculation and vdW-DF calculation to assess the quality of the results.
- The spin polarised calculations are possible, but strictly speaking the non-local vdW correlation is not defined for spin-polarized systems. For a spin-polarized calculation the non-local vdW correlation energy is evaluated on the sum of the spin-up and spin-down densities.
- The evaluation of the vdW energy requires some additional time. Most of it is spent on performing FFTs to evaluate the energy and potential. Thus the additional time is determined by the number of FFT grid points in the calculation, basically the size of the simulation cell. It is almost independent on the number of the atoms in the cell. Thus the relative cost of the vdW-DF method depends on the "filling" of the cell and increases with the amount of vacuum in the cell. The relative increase is high for isolated molecules in large cells, but small for solids in smaller cells with many k-points.
Related Tags and Sections
- ↑ a b c M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004).
- ↑ a b G. Román-Pérez, J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009).
- ↑ a b c d J. Klimeš, D. R. Bowler, and A. Michaelides, J. Phys.: Cond. Matt. 22, 022201 (2010).
- ↑ a b c K. Lee, E. D. Murray, L. Kong, B. I. Lundqvist, and D. C. Langreth, Phys. Rev. B 82, 081101 (2010).
- ↑ a b c d J. Klimeš, D. R. Bowler, and A. Michaelides, Phys. Rev. B 83, 195131 (2011).
- ↑ T. Thonhauser, V. R. Cooper, L. Shen, A. Puzder, P. Hyldgaard, and D. C. Langreth, Phys. Rev. B 76, 125112 (2007).
- ↑ I. Hamada, Phys. Rev. B 89, 121103 (2014).
- ↑ H. Peng, Z.-H. Yang, J. P. Perdew, and J. Sun, Phys. Rev. X 6, 041005 (2016).