A Self-Consistent Charge Density-Functional Based Tight-Binding Scheme for Large Biomolecules

(a) Department of Physics, Harvard University, Cambridge MA 02138, USA(b) Theoretische Physik, Universita¨t Paderborn, D-33098 Paderborn, Germany(c) Molekulare Biophysik, Deutsches Krebsforschungszentrum, D-69120 Heidelberg,Germany(Received August 10, 1999)A common feature of traditional tight-binding (TB) methods is the non-self-consistent solution ofthe eigenvalue problem of a Hamiltonian operator, represented in a minimal basis set. These TBschemes have been applied mostly to solid state systems, containing atoms with similar electrone-gativities. Recently self-consistent TB schemes have been developed which now allow the treat-ment of systems where a redistribution of charges, and the related detailed charge balance be-tween the atoms, become important as e.g. in biological systems. We discuss the application ofsuch a method, a self-consistent charge density-functional based TB scheme (SCC-DFTB), to bio-logical model compounds. We present recent extensions of the method: (i) The combination of thetight binding scheme with an empirical force field, that makes large scale simulations with severalthousand atoms possible. (ii) An extension which allows a quantitative description of weak-bond-ing interactions in biological systems. The latter include an improved description of hydrogenbonding achieved by extending the basis set and improved molecular stacking interactionsachieved by incorporating the dispersion contributions empirically. In applying the method, we pre-sent benchmarks for conformational energies, geometries and frequencies of small peptides andcompare with ab initio and semiempirical quantum chemistry data. These developments provide afast and reliable method, which can handle large scale quantum molecular dynamic simulations inbiological systems.