Grzegorz Mazur

Application of the dressed time‐dependent density functional theory for the excited states of linear polyenes

Dressed Time‐Dependent Density Functional Theory (Maitra et al., J Chem Phys 2004, 120, 5932) is applied to selected linear polyenes. Limits of validity of the approximation are briefly discussed. The implementation strategy is described. Results for the 21Bu and 21Ag states of selected linear polyenes are presented and compared with accessible experimental and theoretical results.

Quantum chemical results as input for solid state calculations: charge transfer states in molecular crystals ☆

The difference I−A between the ionization potential and electron affinity of a molecule is one of the input parameters necessary for calculating the energies of charge transfer states in molecular crystals. In the present paper, the performance of the DFT methodology with several alternative forms of the exchange-correlation potential is tested in this context. Calculations are carried out for polyacenes to select the best potentials and to set up a scheme for correcting systematic errors of the approach, which is subsequently used to calculate the relevant quantity for sexithiophene and perylenetetracarboxylic dianhydride (PTCDA). Combined with independent calculations of the electrostatic energy of charge pairs in these crystals, the results are confronted with CT state energies observed in electro-absorption spectroscopy.

Theoretical calculations of the electroabsorption spectra of perylenetetracarboxylic dianhydride

A three-dimensional analog of the Merrifield model is proposed to study excitons in solid perylenetetracarboxylic dianhydride (PTCDA). Its relevant parameters are estimated by independent calculations, and finally determined by fitting the experimental absorption and electroabsorption spectra of PTCDA films; the optimum values obtained from the fit correspond well with the calculated values. The results justify description of PTCDA excitons within a one-dimensional model, as proposed in the literature. The calculated spectra are generally in good agreement with the experimental ones, with the exception of the directional properties of the electroabsorption signal. A possible reason for this discrepancy is suggested. Generally, the results highlight the prominent role of charge-transfer excitons in the electroabsorption spectrum of PTCDA by showing the experimental features that are difficult to explain without invoking these states.

Charge transfer excitons in perylenetetracarboxylic dianhydride – microelectrostatic calculations

Abstract The energies of charge transfer (CT) states of the perylenetetracarboxylic dianhydride (PTCDA) crystal are evaluated by means of the Fourier transform (FT) and self-consistent polarization field (SCPF) methods. For the nearest-neighbour CT state the results support the estimates of other authors, based on the fit of the electro-absorption spectrum. The calculations also suggest that, owing to the peculiar structure of the PTCDA crystal, other CT states are located in the same energy range, which should be taken into account in future extensions of the existing theoretical models.

Band gap and binding energies of charge-transfer excitons in organic molecular crystals

Abstract The observed difference of 0.2–0.3 eV between the optical and electric band gap in organic solids was commonly attributed to vibrational relaxation. The recently calculated values [I.V. Brovchenko, Chem. Phys. Lett. 278 (1997) 355] of the lattice relaxation energy in the vicinity of a charge carrier or a charge-transfer state never exceed 0.02–0.03 eV, demonstrating that the conventional rationalization of the difference is no longer tenable. An alternative explanation is proposed where the difference is interpreted in terms of charge delocalization and the past controversies are attributed to inconsistencies in treating this contribution to energy.

Automatized Parameterization of DFTB Using Particle Swarm Optimization.

We present a novel density-functional tight-binding (DFTB) parametrization toolkit developed to optimize the parameters of various DFTB models in a fully automatized fashion. The main features of the algorithm, based on the particle swarm optimization technique, are discussed, and a number of initial pilot applications of the developed methodology to molecular and solid systems are presented.

An improved SCPF scheme for polarization energy calculations.

Convergence of the Self‐Consistent Polarization Field (SCPF) method of polarization energy calculations for organic molecular materials is analysed. Use of the Conjugate Gradients method for solving the SCPF equations is proposed. Efficiency of both the original and the newly proposed approach is compared for selected model systems. Brief discussion of the factors influencing the performance of Krylov‐space‐based methods for polarization energy calculations is presented.

A neural-network controlled dynamic evolutionary scheme for global molecular geometry optimization

A neural-network controlled dynamic evolutionary scheme for global molecular geometry optimization A novel, neural network controlled, dynamic evolutionary algorithm is proposed for the purposes of molecular geometry optimization. The approach is tested for selected model molecules and some molecular systems of importance in biochemistry. The new algorithm is shown to compare favorably with the standard, statically parametrized memetic algorithm.

Charge-pair states in organic molecular crystals: localized vs. delocalized description

Abstract The Merrifield model of electronic excitations in a linear molecular crystal is applied to investigate the relation between the bound electron–hole pairs (charge transfer excitons) and the corresponding continuum of unbound states. It is demonstrated that, contrary to the conventional explanation, the difference of 0.2–0.3 eV between the electric and optical band gap observed for polyacene crystals is a consequence of charge delocalization rather than lattice relaxation. The results also suggest that in some molecular crystals and polymers, moderate structural disorder may strengthen the photoconductive response instead of suppressing it, which suggests a new strategy in the search for efficient photoconductors.

Partial atomic multipoles for internally consistent microelectrostatic calculations

An extension of the extant microelectrostatic methodologies, based on the concept of distributed generalized polarizability matrix derived from the Coupled Perturbed Hartree–Fock (CPHF) equations, is proposed for self‐consistent calculation of charge carrier and charge‐transfer (CT) state electrostatic energies in molecular solids, including the doped, defected and disordered ones. The CPHF equations are solved only once and the generalized molecular polarizability they yield enables low cost iterations that mutually adjust the molecular electronic distributions and the local electric field in which the molecules are immersed. The approach offers a precise picture of molecular charge densities, accounting for atomic partial multipoles up to order 2, which allows one to reproduce the recently reported large charge‐quadrupole contributions to CT state energies in low‐symmetry local environments. It is particularly well suited for repetitive calculations for large clusters (up to 300,000 atoms), and may potentially be useful for describing electrostatic solvent effects.