Pulse-Induced Dynamics of a Charge-Transfer Complex from First Principles.

The ultrafast dynamics of charge carriers in organic donor-acceptor interfaces are of primary importance to understanding the fundamental properties of these systems. In this work, we focus on a charge-transfer complex formed by quaterthiophene p-doped by tetrafluoro-tetracyanoquinodimethane and investigate electron dynamics and vibronic interactions also at finite temperatures by applying a femtosecond pulse in resonance with the two lowest energy excitations of the system with perpendicular and parallel polarization with respect to the interface. The adopted ab initio formalism based on real-time time-dependent density-functional theory coupled to Ehrenfest dynamics enables monitoring the dynamical charge transfer across the interface and assessing the role played by the nuclear motion. Our results show that the strong intermolecular interactions binding the complex already in the ground state influence the dynamics, too. The analysis of the nuclear motion involved in these processes reveals the participation of different vibrational modes depending on the electronic states stimulated by the resonant pulse. Coupled donor-acceptor modes mostly influence the excited state polarized across the interface, while intramolecular vibrations in the donor molecule dominate the excitation in the orthogonal direction. The results obtained at finite temperatures are overall consistent with this picture, although thermal disorder contributes to slightly decreasing interfacial charge transfer.

Accurate interaction energies by spin component scaled Möller-Plesset second order perturbation theory calculations with optimized basis sets (SCS-MP2mod): Development and application to aromatic heterocycles.

The Spin Component Scaled (SCS) MP2 method using a reduced and optimized basis set (SCS-MP2mod) is employed to compute the interaction energies of nine homodimers, formed by aromatic heterocyclic molecules (pyrrole, furan, thiophene, oxazole, isoxazole, pyridine, pyridazine, pyrimidine, and pyrazine). The coefficients of the same-spin and opposite-spin correlation energies and the Gaussian type orbitals (GTO) polarization exponents of the 6-31G** basis set are simultaneously optimized in order to minimize the energy differences with respect to the coupled-cluster with single, double and perturbative triples excitations [CCSD(T)] reference interaction energies, extrapolated to a complete basis set. It is demonstrated that the optimization of the spin scale factors leads to a noticeable improvement of the accuracy with a root mean square deviation less than 0.1 kcal/mol and a largest unsigned deviation smaller than 0.25 kcal/mol. The pyrrole dimer provides an exception, with a slightly higher deviation from the reference data. Given the high benefit in terms of computational time with respect to the CCSD(T) technique and the small loss of accuracy, the SCS-MP2mod method appears to be particularly suitable for extensive sampling of intermolecular potential energy surfaces at a quantum mechanical level. Within this framework, a transferability test of the SCS-MP2mod parameters to a benchmark set of this class of molecules is very promising as the reference interaction energies of several heterocyclic aromatic heterodimers were reproduced with a standard deviation of 0.30 kcal/mol. The SCS-MP2mod remarkably outperforms the value of 1.95 kcal/mol obtained with standard MP2/6-31G**.

Development and Validation of Quantum Mechanically Derived Force-Fields: Thermodynamic, Structural, and Vibrational Properties of Aromatic Heterocycles.

A selection of several aromatic molecules, representative of the important class of heterocyclic compounds, has been considered for testing and validating an automated Force Field (FF) parametrization protocol, based only on Quantum Mechanical data. The parametrization is carried out separately for the intra- and intermolecular contributions, employing respectively the Joyce and Picky software packages, previously implemented and refined in our research group. The whole approach is here automated and integrated with a computationally effective yet accurate method, devised very recently ( J. Chem. THEORY: Comput., 2018, 14, 543-556) to evaluate a large number of dimer interaction energies. The resulting quantum mechanically derived FFs are then used in extensive molecular dynamics simulations, in order to evaluate a number of thermodynamic, structural, and dynamic properties of the heterocycle's gas and liquid phases. The comparison with the available experimental data is good and furnishes a validation of the presented approach, which can be confidently exploited for the design of novel and more complex materials.

Interaction Energy Landscapes of Aromatic Heterocycles through a Reliable yet Affordable Computational Approach.

Noncovalent interactions between homodimers of several aromatic heterocycles (pyrrole, furan, thiophene, pyridine, pyridazine, pyrimidine, and pyrazine) are investigated at the ab initio level, employing the Möller-Plesset second-order perturbation theory, coupled with small Gaussian basis sets (6-31G* and 6-31G**) with specifically tuned polarization exponents. The latter are modified using a systematic and automated procedure, the MP2mod approach, based on a comparison with high level CCSD(T) calculations extrapolated to a complete basis set. The MP2mod results achieved with the modified 6-31G** basis set show an excellent agreement with CCSD(T)/CBS reference energies, with a standard deviation less than 0.3 kcal/mol. Exploiting its low computational cost, the MP2mod approach is then used to explore sections of the intermolecular energy of the considered homodimers, with the aim of rationalizing the results. It is found that the direct electrostatic interaction between the monomers electron clouds is at the origin of some observed features, and in many cases multipoles higher than dipole play a relevant role, although often the interplay with other contributions to the noncovalent forces (as for instance induction, π-π or XH-π interactions) makes a simple rationalization rather difficult.