DFT-D2 method

In the D2 method of Grimme [127], the correction term takes the form:

where the summations are over all atoms and all translations of the unit cell , the prime indicates that for , denotes the dispersion coefficient for the atom pair , is distance between atom located in the reference cell =0 and atom in the cell , and the term is a damping function whose role is to scale the force field such as to minimize contributions from interactions within typical bonding distances. In practice, the terms in eq. 6.87 corresponding to interactions over distances longer than a certain suitably chosen cutoff radius contribute only negligibly to and can be ignored. Parameters and are computed using the following combination rules:

(6.88) |

(6.89) |

the values of and are tabulated for each element and are insensitive to the particular chemical situation (for instance, for carbon in methane takes exactly the same value as that for C in benzene within this approximation). In the original method of Grimme [127], Fermi-type damping function is used:

whereby the global scaling parameter has been optimized for several different DFT functionals such as PBE (), BLYP (), and B3LYP (). The parameter is usually fixed at 1.00. The DFT-D2 method can be activated by setting

VDW_RADIUS |
= 50.0 | cutoff radius (Å) for pair interactions |

VDW_S6 |
= 0.75 | global scaling factor |

(available in VASP.5.3.4 and later) | ||

VDW_SR |
= 1.00 | scaling factor |

(available in VASP.5.3.4 and later) | ||

VDW_SCALING |
=0.75 | the same as VDW_S6 |

(obsolete as of VASP.5.3.4) | ||

VDW_D |
= 20.0 | damping parameter |

VDW_C6 |
= [real array] | parameters ( ) for each species |

defined in POSCAR | ||

VDW_R0 |
= [real array] | parameters (Å) for each species |

defined in POSCAR | ||

LVDW_EWALD |
= .FALSE..TRUE. | compute lattice summation in expression |

by means of Ewald's summation - noyes | ||

(available in VASP.5.3.4 and later) |

The performance of PBE-D2 method in optimization of various crystalline systems has been tested systematically in J. Phys. Chem. A 114, 11814 (2010).

IMPORTANT NOTES:

- the defaults for
`VDW_C6`and`VDW_R0`are defined only for elements in the first five rows of periodic table (i.e. H-Xe) - if the system contains other elements the user must define these parameters in INCAR. - the defaults for parameters controlling damping function (
`VDW_S6`,`VDW_SR`,`VDW_D`) are available only for the PBE functional. If functional other than PBE is used in DFT+D2 calculation, the value of`VDW_S6`(or`VDW_SCALING`in versions before VASP.5.3.4) must be defined in INCAR. - as of VASP.5.3.4, the default value for
`VDW_RADIUS`has been increased from 30 to 50 Å. - Ewald's summation in calculation (controlled via
`LVDW_EWALD`) implemented according to Ref. [132] is available as of VASP.5.3.4