Benchmarking Aspects of Ab Initio Fragment Models for Accurate Excimer Potential Energy Surfaces

Abstract

While Coupled-Cluster methods have been proven to provide an accurate description of excited electronic states, the scaling of the computational costs with the system size limits the degree for which these methods can be applied. In this work different aspects of fragment-based approaches are studied on noncovalently bound molecular complexes with interacting chromophores of the fragments, such as π-stacked nucleobases. The interaction of the fragments is considered at two distinct steps. First, the states localized on the fragments are described in the presence of the other fragment(s); for this we test two approaches. One method is founded on QM/MM principles, only including the electrostatic interaction between the fragments in the electronic structure calculation with Pauli repulsion and dispersion effects added separately. The other model, a Projection-based Embedding (PbE) using the Huzinaga equation, includes both electrostatic and Pauli repulsion and only needs to be augmented by dispersion interactions. In both schemes the extended Effective Fragment Potential (EFP2) method of Gordon et al. was found to provide an adequate correction for the missing terms. In the second step, the interaction of the localized chromophores is modeled for a proper description of the excitonic coupling. Here the inclusion of purely electrostatic contributions appears to be sufficient: it is found that the Coulomb part of the coupling provides accurate splitting of the energies of interacting chromophores that are separated by more than 4 Å.

Analysis of Site Energies and Excitonic Couplings: The Role of Symmetry and Polarization

“…There is a variety of theoretical methods developed for calculating local − and nonlocal − excited states in large molecular systems. There is an ongoing work to model the excitonic coupling ,− and the effect of environment on it. − Fragment-based methods are naturally conducive to handling the excitonic coupling by treating chromophore as fragments. There are other low-scaling methods for calculations of excited states. − …”

Projected Atomic Orbitals As Optimal Virtual Space for Excited State Projection-Based Embedding Calculations

“…Using the original SPADE algorithm for the virtual orbitals we observed, however, that without any further adjustments, the resulting virtual space can be severely distorted compared to that of an isolated monomer. As suggested in ref , the distortion can be reduced by extending the space by some environment orbitals that show the largest overlap with the atomic orbitals centered on the active subsystem (called the “extended SPADE” approach hereafter). Nevertheless, the effective construction of appropriate virtual orbitals, especially if diffuse basis functions are present, remains a challenge in PbE applications.…”

“…These are avoided in top-down projector-based embedding techniques which maintain the orthogonality of the subsystems by applying appropriate projectors; however, a low-level calculation on the entire system is necessary. While for noncovalently interacting systems the nonadditive terms are less crucial, for higher accuracy and for generalizability, top-down projection-based embedding (PbE) methods seem more suited for the description of multichromophore systems …”

“…Since only the electrons defining the density on the active fragment(s) are used in these calculations, a significant reduction of the cost of the calculation is achieved. Details of the PbE workflow used in this study are described in Section S1.3 (see also refs and ).…”

“…Nevertheless, the original formulations of PbE , leave the space of virtual orbitals intact, i.e., that of the original supersystem, which poses serious limitations on the applicability of the higher-level ab initio methods due to the unfavorable scaling of the computational cost with the size of the virtual space . In addition, the untruncated virtual space tendentially leads to the appearance of artifactual low-lying charge transfer (CT) states, often rendering the identification of local excited states impossible. Several approaches have been investigated to deal with this problem by truncating the virtual space in a reasonable way, including the absolutely localized embedding schemes of Chulhai and Goodpaster , that avoid the issue entirely by using monomer basis sets, or the basis set truncation method for PbE by Bennie et al which relies on net Mulliken population criteria.…”