Three-Body Expansion and Generalized Dynamic Fragmentation Improve the Fragment Molecular Orbital-Based Molecular Dynamics (fMO-MD), An ab Initio MD Method

Abstract

The Fragment Molecular Orbital-based Molecular Dynamics (FMO-MD) is an ab initio MD method suitable for simulation of large molecular systems [1]. FMO-MD was improved by the introduction of the three-body extension (FMO3, [2]) and the generalized dynamic fragmentation, namely configuration-dependent redefinition of fragments during FMO-MD. An analytical energy gradient, that is, minus the force, was derived for FMO3 and was implemented to realize FMO3-MD. An algorithm of generalized dynamic fragmentation was devised to treat each covalent-bonded and, optionally, hydrogen-bonded atom cluster as a fragment. The new algorithms were tested by performing MO-MD, based on the molecular orbital method, FMO2-MD, based on two-body extension, and FMO3-MD simulations of (H2O)32 and H+(H2O)32. FMO3-MD gave a precision comparable to that of MO-MD, while FMO2-MD resulted in lower precision, especially in H+(H2O)32. The tests also showed that the generalized dynamic fragmentation scheme treated the H+ transfer reaction gracefully in H+(H2O)32. These results of the test simulations revealed the feasibility of FMO3-MD and the generalized dynamic fragmentation.[1] Y. Komeiji et al., Chem. Phys. Lett. 372 (2003) 342., J. Comput. Chem. 30 (2009) 40.[2] D. G. Fedorov, K. Kitaura, J. Chem. Phys. 120 (2004) 6832, Chem. Phys. Lett. 433 (2006) 182.