DFT-D2

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

$E_{\mathrm{disp}} = -\frac{1}{2} \sum_{i=1}^{N_{at}} \sum_{j=1}^{N_{at}} \sum_{\mathbf{L}} ^{\prime} \frac{C_{6ij}}{r_{ij,L}^{6}} f_{d,6}({r}_{ij,L})$

where the summations are over all atoms $N_{at}$ and all translations of the unit cell ${L}=(l_1,l_2,l_3)$. The prime indicates that $i\not=j$ for ${L}=0$, $C_{6ij}$ denotes the dispersion coefficient for the atom pair $ij$, ${r}_{ij,L}$ is the distance between atom $i$ located in the reference cell $L=0$ and atom $j$ in the cell $L$ and the term $f(r_{ij})$ is a damping function whose role is to scale the force field such as to minimize the contributions from interactions within typical bonding distances. In practice, the terms in the equation for $E_{\mathrm{disp}}$ corresponding to interactions over distances longer than a certain suitably chosen cutoff radius contribute only negligibly to $E_{\mathrm{disp}}$ and can be ignored. Parameters $C_{6ij}$ and $R_{0ij}$ are computed using the following combination rules:

$C_{6ij} = \sqrt{C_{6ii} C_{6jj}}$

and

$R_{0ij} = R_{0i}+ R_{0j}.$

The values for $C_{6ii}$ and $R_{0i}$ are tabulated for each element and are insensitive to the particular chemical situation (for instance, $C_6$ for carbon in methane takes exactly the same value as that for C in benzene within this approximation). In the original method of Grimme, a Fermi-type damping function is used:

$f_{d,6}(r_{ij}) = \frac{s_6}{1+e^{-d(r_{ij}/(s_R\,R_{0ij})-1)}}$

whereby the global scaling parameter $s_6$ has been optimized for several different DFT functionals such as PBE ($s_6=0.75$), BLYP ($s_6=1.2$) and B3LYP ($s_6=1.05$). The parameter $s_R$ is usually fixed at 1.00. The DFT-D2 method can be activated by setting IVDW=1|10 or by specifying LVDW=.TRUE. (this parameter is obsolete as of VASP.5.3.3). Optionally, the damping function and the vdW parameters can be controlled using the following flags (the default values are listed):

• VDW_RADIUS=50.0 cutoff radius (in $\AA$) for pair interactions
• VDW_S6=0.75 global scaling factor $s_6$ (available in VASP.5.3.4 and later)
• VDW_SR=1.00 scaling factor $s_R$ (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 $d$
• VDW_C6=[real array] $C_6$ parameters ($\mathrm{Jnm}^{6}\mathrm{mol}^{-1}$) for each species defined in the POSCAR file
• VDW_R0=[real array] $R_0$ parameters ($\AA$) for each species defined in the POSCAR file
• LVDW_EWALD=.FALSE. decides whter lattice summation in $E_{disp}$ expression by means of Ewald's summation is computed (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 reference .\\

noindent 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 the damping function (VDW_S6, VDW_SR, VDW_D) are available only for the PBE functional. If a 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 $\AA$.

• Ewald's summation in the calculation of $E_{disp}$ calculation (controlled via LVDW_EWALD) is implemented according to reference and is available as of VASP.5.3.4.

Related Tags and Sections

IVDW, IALGO

References

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