GDB-9-Ex: Quantum chemical prediction of UV/Vis absorption spectra for GDB-9 molecules

GDB-9-Ex: Quantum chemical prediction of UV/Vis absorption spectra for GDB-9 molecules Massimiliano Lupo Pasini, Pilsun Yoo, Kshitij Mehta, Stephan Irle Python, GDB-9, Time-Dependent Density-Functional Tight-Binding (TD-DFTB), Predicting Excited States Molecular Properties We performed calculations of electronic excitation energies and associated oscillator strengths based on the time-dependent density-functional tight-binding (TD-DFTB) method [1]. The SMILES (Simplified molecular-input line-entry system) strings of the molecules from the GDB-9 database [2] were converted to a 3D atomistic structure and stored in a PDB file after preliminary geometry optimization using the Merck Molecular Force Field (MMFF94) in RDKit [3,4]. The primary information stored in the PDB file archive consists of Cartesian coordinates for each atom of the molecule in their 3D location in space, along with summary information about the structure, sequence, and experiment. We then performed molecular geometry optimization using the density-functional tight-binding (DFTB) method [5] in the electronic ground state, followed by single-point excited states calculations, as described below. The computed excitation energies and associated oscillator strengths can be converted to predict UV/Vis absorption spectra, where excitation energies correspond to absorption peak positions, and oscillator strengths are a good measure of the probability of absorption of visible or UV light in transitions between electronic ground and excited states. The conversion of SMILES strings to 3D Cartesian coordinates of fully DFTB-optimized molecules was successful for 96,766 molecules, for which both geometry optimizations and excited states calculations were successful. The DFTB method [5] is an approximation to density functional theory (DFT), utilizing a minimal basis set in conjunction with a two-center approximation to the electronic Hamiltonian and overlap matrix elements. The DFTB total energy is the sum of an electronic and a repulsive energy contribution, and their calculation requires optimized electronic parameters and diatomic repulsive potential energy functions. When charge transfer or polarization between atoms are explicitly considered, the total DFTB electronic energy E is expressed as a Taylor expansion of the in terms of density fluctuations Î´Ï around atomic reference densities Ï_0 as [4a] E[Ï] = E_0 [Ï_0 ] + E_1 [Ï_0, δÏ] + E_2 [Ï_0, (δÏ)^2 ] + E_3 [Ï_0, (δÏ)^3 ] + ⋯ . In the DFTB formulation, termination of this series at various orders is termed as different DFTB “flavors†(DFTB1, DFTB2, etc.) which corresponds to adding correction terms for higher accuracies in the interatomic Coulombic interaction. All DFTB calculations were performed using the DFTB+ code6 (version 21.2) and the wrapper for DFTB+ in the Atomic Simulation Environment (ASE) [7], which performed an internal conversion of Cartesian coordinates from PDB to the .gen file format. For the geometry optimizations on the electronic ground state potential energy surface of the molecules, we have chosen the third-order DFTB (DFTB3) method [5c] and employed the matching 3ob set of electronic parameters and repulsive potentials [8]. The empirical γ-damping for hydrogen bond correction, and Grimme's D [3] empirical dispersion correction with Becke-Johnson damping (D3(BJ)) [9] dispersion correction was included to improve the description of noncovalent interactions. For excited states single-point energy calculations, we employed the TD-DFTB method in conjunction with the DFTB2 method [5b] and the matching mio [5b,10] and halorg [11] parameter sets. We opted to request the simultaneous calculation of 50 excited singlet states to investigate sufficient number of excited states, based on linear response theory using the Casida equation and the ARPACK diagonalizer. FILES dftb-uv_2d.py: Script to convert SMILES to a smiles.pdb file, and the ASE wrapper to generate the geo_end.gen and dftb_in.hsd files for DFTB calculations. geo_end.gen detailed.out band.out EXC.DAT smiles.pdb REFERENCES [1] Niehaus, T. A.; Suhai, S.; Della Salla, F.; Lugli, P.; Elstner, M.; Seifert, G.; Frauenheim, Th. Tight-binding approach to time-dependent density-functional response theory. Phys. Rev. B, 2001, 63, 085108/1-9. [2] a) Ramakrishnan, R.; Dral, P. O.; Rupp, M.; von Lilienfeld, O. A. Quantum Chemistry Structures and Properties of 134 Kilo Molecules. Sci. Data 2014, 1, 140022, DOI: 10.1038/sdata.2014.22; b) Ruddigkeit, L.; van Deursen, R.; Blum, L. C.; Reymond, J.-L. 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