In the first configuration the binding between the base pairs is dominated by hydrogen bonding. The figure reports binding energies obtained from PBE and an accurate reference method for DNA base pairs in two different configurations. Figure 3 illustrates a more “real world” example, where an accurate description of dispersion is critical. However, it has become increasingly apparent that dispersion can contribute significantly to the binding of many other types of materials, such as biomolecules, adsorbates, liquids, and solids. The binding of noble gases is a textbook dispersion bonded system. for a binding curve between two Kr atoms. 2 for one of the most widely used GGAs-the Perdew-Burke-Ernzerhof (PBE) functional 20 20. Since the overlap decays exponentially with the interatomic separation, so too does any binding. The consequence for two noble gas atoms, for example, is that these functionals give binding or repulsion only when there is an overlap of the electron densities of the two atoms. Standard XC functionals do not describe dispersion because: (a) instantaneous density fluctuations are not considered and (b) they are “short-sighted” in that they consider only local properties to calculate the XC energy. The leading term of such an interaction is instantaneous dipole-induced dipole which gives rise to the well known −1/ r 6 decay of the interaction energy with interatomic separation r. (Data from Web of Knowledge, July 2012 for the years 1991–2011.)ĭispersion can be viewed as an attractive interaction originating from the response of electrons in one region to instantaneous charge density fluctuations in another. The total number of studies performed is difficult to establish precisely however, an estimate is made here by illustrating the number of papers that cite at least one of 16 seminal works in the field (Refs. The number of dispersion corrected DFT studies has increased considerably in recent years. Figure 1 underlines this point, where it can be seen that over 800 dispersion-based DFT studies were reported in 2011 compared to fewer than 80 in the whole of the 1990s.įIG. The “lack” of dispersion forces – often colloquially referred to as van der Waals (vdW) forces-is one of the most significant problems with modern DFT and, as such, the quest for DFT-based methods which accurately account for dispersion is becoming one of the hottest topics in computational chemistry, physics, and materials science. One prominent example is the inability of “standard” XC functionals to describe long-range electron correlations, otherwise known as electron dispersion forces by standard XC functionals we mean the local density approximation (LDA), generalized gradient approximation (GGA) functionals or the hybrid XC functionals. However, there are situations where the approximate form of the XC functional leads to problems. These interactions are approximated with so-called exchange-correlation (XC) functionals and much of the success of DFT stems from the fact that XC functionals with very simple forms often yield accurate results. Although DFT is in principle exact, in practice approximations must be made for how electrons interact with each other.
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