## Saturday, April 23, 2016

### Consistent structures and interactions by density functional theory with small atomic orbital basis sets

Stefan Grimme, Jan Gerit Brandenburg, Christoph Bannwarth, and Andreas Hansen (2015)
Contributed by Jan Jensen

B3LYP/6-31G* is still the de facto default level of theory for geometry optimizations of large-ish (ca 50-200 atoms) molecules and this paper introduces a cheaper, more accurate replacement called PBEh-3c. PBEh-3c is a basically PBE0/def2-SV(P) with added dispersion and BSSE corrections, where both functional and basis set has been modified slightly (for B-Ne in the case of the basis set).

As the authors write
Most striking is the roughly “MP2-quality” (or slightly better) obtained for the non-covalent complexes in the S22/S66 sets and equilibrium structures ($B_e$ values) for medium-sized organic molecules in the ROT34 set.
For example, the S22 and S66 geometries can be reproduced with an RMSD of 0.08 and 0.05 Å, respectively.  But this method is aimed at the entire periodic table and bond lengths between heavier atoms are also tested and well reproduced.

Though not specifically designed for it the method is also tested for intermolecular interaction energies, reaction energies, barrier heights.  Here PBEh-3c doesn't always outperform B3LYP/def2-SV(P) or M06-2X/def2-SV(P), but when it does it's typically a very significant improvement.  For example, the S30L (which includes host-guest complexes with multiple hydrogen bonds and/or charged systems) interaction energies are reproduced with an MAD of 3.4 kcal/mol, compared to 7.4 and 25.9 kcal/mol for M06-2X/def2-SV(P) and B3LYP/def2-SV(P), respectively.  3.3 kcal/mol may still sound like a lot but, for comparison, the corresponding MAD for PW6B95-D3/def2-QZVP(D) is 2.5 kcal/mol.