Abstract
The DFTB method, as well as its self-consistent charge corrected variant SCC-DFTB, has widened the range of applications of fundamentally well established theoretical tools. As an approximate density-functional method, DFTB holds nearly the same accuracy, but at much lower computational costs, allowing investigation of the electronic structure of large systems which can not be exploited with conventional ab initio methods. In the present paper the fundaments of DFTB and SCC-DFTB and inclusion of London dispersion forces are reviewed. In order to show an example of the DFTB applicability, the zwitterionic equilibrium of glycine in aqueous solution is investigated by molecular-dynamics simulation using a dispersion-corrected SCC-DFTB Hamiltonian and a periodic box containing 129 water molecules, in a purely quantum-mechanical approach.
Highlights
Density functional theory (DFT) methods are the standard and the most used theoretical techniques for electronic structure calculations.[1,2,3,4,5] The advent of the generalized gradient approximation (GGA) for the exchange-correlation functional enhanced the DFT accuracy[6] and the predicted molecular structures, relative energies and frequencies are nearly comparable to the second order Møller-Plesset perturbation theory (MP2) method, with remarkable success to treat transition metal complexes.[7]
Chemical property estimates based on DFT are well established, and even optical properties are accessible through the generalization to time-dependent DFT,[7,12,13] a method which is nowadays implemented in many different computer codes
The Density-functional tight-binding (DFTB) method attends these three requirements with the additional advantage of completely avoiding any empirical parameterization, since the Hamiltonian and overlap matrices are calculated using atom-like valenceorbitals which are derived from DFT
Summary
Density functional theory (DFT) methods are the standard and the most used theoretical techniques for electronic structure calculations.[1,2,3,4,5] The advent of the generalized gradient approximation (GGA) for the exchange-correlation functional enhanced the DFT accuracy[6] and the predicted molecular structures, relative energies and frequencies are nearly comparable to the second order Møller-Plesset perturbation theory (MP2) method, with remarkable success to treat transition metal complexes.[7]. Biosystems, adsorption processes, nanostructures, molecular dynamics, clusters and aggregates with thousands of atoms, self-assembling systems, nanoreactors and supramolecular chemistry are some of the fields in which ab initio methods cannot be used with adequate chemical models. For this range of systems, semi-empirical methods seem to have their applicability. As an example of application, the zwitterionic and neutral forms of glycine in aqueous solution are discussed in terms of fully quantum mechanical molecular dynamics of this molecule in water
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