Combining Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB)with Molecular Mechanics by the Generalized Hybrid Orbital (GHO)Method

摘要

In combined quantum mechanical and molecular mechanical (QM/MM)calculations with the QM/MM boundary at a covalent bond,the generalized hybrid orbital (GHO)method has been shown to provide a well balanced and stable connection between the QM and MM regions.The GHO method has previously been developed for semiempirical molecular orbital methods based on neglect of diatomic differential overlap (GHO-NDDO)and for the ab initio Hartree-Fock level (GHO-AIHF).In the present work,we formulate the GHO algorithm and its analytical gradients for treating the QM subsystem by the self-consistent-charge density-functional tight-binding (SCC-DFTB)theory.To obtain a good description of the bond length at the QM/ MM boundary,a parametrized empirical correction term involving the GHO boundary atom and its QM frontier neighbor is added.Geometries and Mulliken charges obtained from GHO-SCC-DFTB calculations are compared to the fully QM results for a set of 18 molecules and ions with various functional groups close to the boundary,and we verified that we reproduced the full C-C stretch potential in ethane and in propanoate.The torsion barrier of n-butane around the central C-C bond is studied with the GHO boundary atom placed at different locations.Finally,the energetics of the method are tested for the proton affinities of a series of 15 alcohols,amines,thiols,and acids.The results indicate that the GHO treatment for combining SCC-DFTB with molecular mechanics is both theoretically robust and satisfactory for practical use.In Supporting Information we present parameters for boundaries that cut through O-C and S-C bonds.