## Saturday, September 27, 2014

### A General Quantum Mechanically Derived Force Field (QMDFF) for Molecules and Condensed Phase Simulations

Stefan Grimme J. Chem. Theory Comput. 2014, ASAP
Contributed by +Jan Jensen

Stefan Grimme has develop a black-box approach by which a molecule-specific polarizable force field that can be automatically generated from the equilibrium geometry, Hessian, atomic partial charges and covalent bond orders computed with standard quantum mechanical methods.  In addition to the molecule-specific parameters there are 47 fixed parameters (listed in the paper) that were parameterized by separate QM calculations and cover all elements up to Radon.

One of the key challenges for any FF parameterization is the parameterization of the torsions (and any inversions) which can be a difficult to automate.  Grimme solves this by automatically extracting all relevant four atom fragments (CABD) including first-shell substituents on atoms C and D, adding H atoms where needed, and performing a 360$^\circ$ energy scan using modified extended Hückel calculations. The resulting PES is fit to the usual $cos$ functional form.  The extended Hückel calculations are modified to avoid double counting 1,4 interaction also included in the non-bonded part of the FF.

So what can you use the QMDFF for?  Well, for example, you can perform a QM calculation on a single conformation of a flexible molecule and use the resulting QMDFF to map out the conformational PES.  A similar calculation on a single solvent molecule allows you to explicitly solvate the molecule.  Similarly, one can search for the lowest energy conformation (and compute frequencies!) of host-guest complexes based on QM calculations on the host and guest.  For very large systems some kind of fragmentation scheme is required.

Finally, the QMDFF also allows for bonds to be broken (but not formed) so it can also be used to predict mass spectra using Grimme's QCEIMS method, which I highlighted earlier.  Since bonds cannot be formed, QMDFF is not a reactive FF like ReaxFF, but it is interesting to think about how one could make it in to one.