This keyword specifies the function which has to be used to dampen atom-atom dispersion energies. Available functions are reported in the following table:

The need for a damping function arises from the fact that
the dispersion energy behaves as *r*^{-6} and becomes physically
unrealistic at small distances *r* where it diverges.

The parameter *R*_{0} in the expression of
MDDRE,
EHFSK and
WY damping functions is the sum of
atomic van der Waals radii. PAMoC employs the vdW radii determined by
Grimme for elements H-Xe [5"Semiempirical
GGA-Type Density Functional Constructed with a Long-Range Dispersion
Correction"

S. Grimme *J. Comput. Chem.* **2006**,
*27*, 1787-1799.].

Liu and Goddard III pointed out that MDDRE, EHFSK and WY damping functions can be represented by a single formula

f_{6}(r)
= {1 + a exp[−b
(r
R_{0}
)m]} n |

with different choices of the parameters *a*, *b*, *m*, and
*n* [8"A Universal Damping Function for
Empirical Dispersion Correction on Density Functional Theory"

Yi Liu,
W. A. Goddard III *Materials Transactions* **2009**, *50*,
1664-1670.], as shown in the following Table.

Keyword value | a |
b |
m |
n |
Description |

1 | -1 | D_{1} | 3 | 2 | MDDRE damping function. |

2 | -1 | D_{2} | 7 | 4 | EHFSK damping function. |

3 | exp(D_{3}) | D_{3} |
1 | -1 | WY damping function. |

The trend of the four damping functions
(MDDRE,
EHFSK,
WY, and
TT) along the distance *r* is shown
in the left figure below, whereas their effects on the
*C*_{6} *r*^{-6}
term for the OH interaction potential is shown in the right figure.

It appears that the damping strength by EHFSK function is between those by funtions MDDRE and WY, with WY function being the strongest.

- (
`AtmParDsp`|`apde`|`adp`) specifies which set of dispersion atomic parameters has to be used for calculation of atom-atom dispersion interaction energies.

- Energies of nonbonded interactions

- "Transferable ab initio intermolecular potentials. 1. Derivation
from methanol dimer and trimer calculations"

W.T.M. Mooij, F.B. van Duijneveldt, J.G.C.M. van Duijneveldt-van de Rijdt, B.P. van Eijck*J. Phys. Chem. A***1999**,*103*, 9872-9882. - "Empirical correction to density functional theory for van der Waals
interactions"

Q. Wu, W. Yang*J. Chem. Phys.***2002**,*116*, 515-524. - "Hydrogen bonding and stacking interactions of nucleic acid base
pairs: A density-functional-theory based treatment"

M. Elstner, P. Hobza, T. Frauenheim, S. Suhai, E. Kaxiras*J. Chem. Phys.***2001**,*114*, 5149-5155. - "Accurate Description of van der Waals Complexes by Density
Functional Theory Including Empirical Corrections"

S. Grimme*J. Comput. Chem.***2004**,*25*, 1463-1473. - "Semiempirical GGA-Type Density Functional Constructed with a
Long-Range Dispersion Correction"

S. Grimme*J. Comput. Chem.***2006**,*27*, 1787-1799. - "Density Functional Theory Augmented with an Empirical Dispersion
Term. Interaction Energies and Geometries of 80 Noncovalent Complexes
Compared with Ab Initio Quantum Mechanics Calculations"

P. Jurecka, J. Cerny, P. Hobza, D. R. Salahub*J. Comput. Chem.***2007**,*28*, 555-569. - "An improved simple model for the van der Waals potential based
on universal damping functions for the dispersion coefficients."

K. T. Tang, J. P. Toennies*J. Chem. Phys.***1984**,*80*, 3726-3741. - "A Universal Damping Function for Empirical Dispersion Correction
on Density Functional Theory"

Yi Liu, W. A. Goddard III*Materials Transactions***2009**,*50*, 1664-1670.