A self-consistent charge density-functional based tight-binding scheme forlarge biomolecules

A common feature of traditional tight-binding (TB) methods is the non-self- consistent solution of the eigenvalue problem of a Hamiltonian operator, re presented in a minimal basis set. These TB schemes have been applied mostly to solid state systems, containing atoms with similar electronegativities. Recently self-consistent TB schemes have been developed which now allow th e treatment of systems where a redistribution of charges, and the related d etailed charge balance between the atoms, become important as e.g. in biolo gical systems. We discuss the application of such a method, a self-consiste nt charge density-functional based TB scheme (SCC-DFTB), to biological mode l compounds. We present recent extensions of the method: (i) The combinatio n of the tight binding scheme with an empirical force field, that makes lar ge scale simulations with several thousand atoms possible. (ii) An extensio n which allows a quantitative description of weak-bonding interactions in b iological systems. The latter include an improved description of hydrogen b onding achieved by extending the basis set and improved molecular stacking interactions achieved by incorporating the dispersion contributions empiric ally. In applying the method, we present benchmarks for conformational ener gies, geometries and frequencies of small peptides and compare with ab init io and semiempirical quantum chemistry data. These developments provide a f ast and reliable method, which can handle large scale quantum molecular dyn amic simulations in biological systems.