6.1.7  DFT  Calculations with Empirical Van der Waals Correction It is well known that DFT does not include dispersion forces. It is possible to  use  a  simple  semi-empirical correction to account for the major parts  of  this  contribution  to  the  energy  – again, due to Stefan Grimme.  11 We have adopted precisely his code and method in this ORCA version.  The  method  is parameterized for the BLYP, BP86, PBE, TPSS, B3LYP  and  B2PLYP functionals. 12 The empirical correction is invoked by the keyword ! VDW.

! PBE VDW TZVPP VeryTightSCF Opt TightOpt Grid6
* xyz 
Li 0 0 0
Li 0 0 2.7
*
 
This  job  gives 274.3 pm without and 269.2 pm with the VDW correction compared  to  the  reference  value  of  267.0 pm. Thus, the empirical correction  is quite successful and Grimme’s calibration work suggests that  this  is  more  generally  true.  However,  his  favorite  B97-D functional  is  not  yet  available  in  ORCA.  Nevertheless, for many systems  like stacked DNA basepairs, hydrogen bond complexes and other weak  interactions  the empirical van der Waals correction may improve substantially  over  the  results  of standard GGA type functionals at essentially no extra cost.