A Hybrid Method for Solutes in Complex Solvents: Density Functional Theory Combined with Empirical Force Fields

M. Eichinger and P. Tavan

Institut für Medizinische Optik, Theoretische Biophysik,
Ludwig-Maximilians-Universität München, Oettingenstr. 67, D-80538 München, Germany

J. Hutter and M. Parrinello

Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany

J. Chem. Phys., 110: 10452-10467, 1999

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Abstract: We present a hybrid method for molecular dynamics simulations of solutes in complex solvents as represented, for example, by substrates within enzymes. The method combines a quantum mechanical QM description of the solute with a molecular mechanics MM approach for the solvent. The QM fragment of a simulation system is treated by ab initio density functional theory DFT based on plane-wave expansions. Long-range Coulomb interactions within the MM fragment and between the QM and the MM fragment are treated by a computationally efficient fast multipole method. For the description of covalent bonds between the two fragments, we introduce the scaled position link atom method SPLAM, which removes the shortcomings of related procedures. The various aspects of the hybrid method are scrutinized through test calculations on liquid water, the water dimer, ethane and a small molecule related to the retinal Schiff base. In particular, the extent to which vibrational spectra obtained by DFT for the solute can be spoiled by the lower quality force field of the solvent is checked, including cases in which the two fragments are covalently joined. The results demonstrate that our QM/MM hybrid method is especially well suited for the vibrational analysis of molecules in condensed phase.

(C) 1999 American Institute of Physics. [S0021-9606(99)71521-7]