### Abstract

- Top of page
- Abstract
- Introduction
- Computational Methods
- Results and Discussion
- Conclusions
- Acknowledgements
- Supporting Information

A benchmark study on all possible density functional theory (DFT) methods in *Gaussian09* is done to locate functionals that agree well with CCSD/aug-cc-pVTZ geometry and Ave-CCSD(T)/(Q-T) interaction energy (*E*_{int}) for small non-covalently interacting molecular dimers in “dispersion-dominated” (class 1), “dipole-induced dipole” (class 2), and “dipole-dipole” (class 3) classes. A DFT method is recommended acceptable if the geometry showed close agreement to CCSD result (RMSD < 0.045) and *E*_{int} was within 80–120% accuracy. Among 382 tested functionals, 1–46% gave good geometry, 13–44% gave good *E*_{int}, while 1–33% satisfied geometry and energy criteria. Further screening to locate the best performing functionals for all the three classes was made by counting the acceptable values of energy and geometry given by each functionals. The meta-generalized gradient approximation (GGA) functional M06L was the best performer with total 14 hits; seven acceptable energies and seven acceptable geometries. This was the only functional “recommended” for at least two dimers in each class. The functionals M05, B2PLYPD, B971, mPW2PLYPD, PBEB95, and CAM-B3LYP gave 11 hits while PBEhB95, PW91B95, Wb97x, BRxVP86, BRxP86, HSE2PBE, HSEh1PBE, PBE1PBE, PBEh1PBE, and PW91TPSS gave 10 hits. Among these, M05, B971, mPW2PLYPD, Wb97x, and PW91TPSS were among the “recommended” list of at least one dimer from each class. Long-range correction (LC) of Hirao and coworkers to exchange-correlation functionals showed massive improvement in geometry and *E*_{int}. The best performing LC-functionals were LC-G96KCIS and LC-PKZBPKZB. Our results predict that M06L is the most trustworthy DFT method in *Gaussian09* to study small non-covalently interacting systems. © 2013 Wiley Periodicals, Inc.