Analytic Gradients for the Effective Fragment Molecular Orbital Method

The analytic gradient for the Coulomb, polarization, exchange-repulsion, and dispersion terms of :the fully integrated effective fragment molecular orbital (EFMO) method is derived and the implementation is discussed. The derivation of the EFMO analytic gradient is more complicated than that for the effective fragment potential (EFP) gradient, because the geometry of each EFP fragment is flexible (not rigid) in the EFMO approach. The accuracy of the gradient is demonstrated by comparing the EFMO analytic gradient with the numeric gradient for several systems, and by assessing the energy conservation during an EFMO NVE ensemble molecular dynamic's simulation of water molecules. In addition to facilitating accurate EFMO geometry optimizations, this allows calculations with flexible EFP fragments to be performed.