Monday, October 28, 2013

Hybrid RHF/MP2 geometry optimizations with the Effective Fragment Molecular Orbital Method

This is a short post on a paper we are submitting to PLOS ONE. It is of course already (and have been for quite some time) available from arXiv:


Anders S. Christensen, Casper Steinmann, Dmitri G. Fedorov, Jan H. Jensen
The frozen domain Effective Fragment Molecular Orbital method (PLoS ONE (2013) 8(4):e60602) is extended to allow for the treatment of a single fragment at the MP2 level of theory. The approach is applied to the conversion of chorismate to phrephenate by chorismate mutase, where the substrate is treated at the MP2 level of theory while the rest of the system is treated at the RHF level. MP2 geometry optimization is found to lower the barrier by up to 3.5 kcal/mol compared to RHF optimzations and ONIOM energy refinement and leads to smoother convergence with respect to basis set for the reaction profile. For double zeta basis sets the increase in CPU time relative to RHF is roughly a factor of two.

Short summary:

We present a new layering scheme for fragment based calculations in GAMESS using the Fragment Molecular Orbital (FMO) and Effective Fragment Molecular Orbital (EFMO), in which the molecular system is divided into four parts - see figure below.

We are then able to optimize the geometry of a reaction complex (H in Fig. 1) in chorismate mutase at the MP2 level while simultaneously optimizing the geometry of the surrounding environment (A in Fig. 1) which is in turn embedded into a polarizable domain (b in Fig. 1) with frozen geometry, which in turn is embedded inside a non-polarizable, frozen-geometry domain (F. in Fig. 1).

We are treating the system at the MP2 level on the reaction complex and Hartree-Fock level on the rest of the system. As an example we calculate the reaction path of conversion of chorismate to prephenate in chorismate mutase.
The increase in computational load is rougly a factor of two, compared to a Hartree-Fock-only calculation.

The method will be available with a new keyword, but the final name is yet to be decided. There will be a blog post on this later if/when I get this in the official GAMESS distribution.

Figure 1: F denotes the frozen domain (green); b denotes the polarizable domain (blue); A denotes the active domain (red); H 2 A denotes fragment H, for which the MP2 energy and gradients are evaluated (yellow).