Automatized Parametrization of SCC-DFTB Repulsive Potentials: Application to Hydrocarbons

Abstract

In this work, we derive and test a new automatized strategy to construct repulsive potentials for the self-consistent charge density functional tight-binding (SCC-DFTB) method. This approach allows one to explore the parameter space in a systematic fashion in order to find optimal solutions. We find that due to the limited flexibility of the SCC-DFTB electronic part, not all properties can be optimized simultaneously. For example, the optimization of heats of formation is in conflict with the optimization of vibrational frequencies. Therefore, a special parametrization for vibrational frequencies is derived. It is shown that the performance of SCC-DFTB can be significantly improved using a more elaborate fitting strategy. A new fit for C and H is presented, which results in an average error of 2.6 kcal/mol for heats of formations for a large set of hydrocarbons, indicating that the performance of SCC-DFTB can be systematically improved also for other elements.

Density functional tight binding

“…The determination of r 0 and r d 0 is an empirical procedure and can be quite involved [13,16], while U a and U d a can be easily computed in principle. For the modified function γ h one additional parameter appears, which is fitted to reproduce the water dimer binding energy [13].…”

“…This holds, for example, for geometries and molecular geometries, or for geometries and vibrational frequencies. However, no parametrization could be derived which approaches DFT accuracy for both energies and vibrational frequencies simultaneously [13,16]. Therefore, different parametrizations have been proposed.…”

“…This is the key to transferability of the parameters. In DFTB, this term is approximated by a sum of pair potentials called repulsive energy term [15] 10) which is either determined by comparison with DFT calculations [8] or fitted to empirical data [16]. The total energy then reads…”

SCC-DFTB Parametrization for Boron and Boranes

“…In order to achieve a balanced description of reactions that conserve the number of shared electron pairs, we shifted the repulsive potentials for H−B and B−B to obtain a consistent overbinding. 41 At a certain cutoff distance r cut , the repulsive potential is then smoothly brought to zero to ensure correct dissociation. For the present parametrization, the cutoff values were chosen to minimize the errors in bonding distances, angles, and vibrations, which lead to the values r cut H−B = 1.36 Å and r cut B−B = 1.99 Å.…”

Nonadiabatic dynamics study of bridged-azobenzene by the time-dependent density functional tight-binding method

“…Charge consistency is achieved with the definition of Mullikan charges and self-consistent steps are run to allow the charge rearrangement between various elements which improves the intermediate molecular charge transfer. A number of reviews have addressed the detailed derivation and formalism of the method [6,7,47], many benchmark studies are conducted to show the great success and limitations of this method [6,[48][49][50][51][52]. SCC-DTFB can be extended to the time-dependent case with Casida's linear response theory [4,5] to treat the excited state.…”