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

🔗 Hybrid RHF/MP2 Geometry Optimizations with the Effective Fragment Molecular Orbital Method

Authors: 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 prephenate 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 optimizations 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.

📄 Full paper: arxiv.org/abs/1305.0676

🔍 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) methods, in which the molecular system is divided into four parts — see Figure 1 below.

This allows us to:

  • Optimize the geometry of a reaction complex (H) in chorismate mutase at the MP2 level.
  • Simultaneously optimize the geometry of the surrounding environment (A) at the Hartree-Fock (HF) level.
  • Embed this environment into a polarizable domain (b) with frozen geometry.
  • Finally embed the polarizable domain into a non-polarizable, frozen domain (F).

As an example, we calculate the reaction path of the conversion of chorismate to prephenate in chorismate mutase.

Despite the increase in computational detail, the added CPU time is only about 2× that of a Hartree-Fock-only calculation.

The method will be available in GAMESS with a new keyword — name TBD. I’ll write a follow-up post if/when it’s included in the official GAMESS distribution.

🧪 Figure 1

  • F: Frozen domain (green)
  • b: Polarizable domain (blue)
  • A: Active domain (red)
  • H ∈ A: Fragment H, for which MP2 energy and gradients are evaluated (yellow)

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).