Multiple Facets of Modeling Electronic Absorption Spectra of Systems in Solution

Abstract

In this Perspective, we outline the essential physicochemical aspects that need to be considered when building a reliable approach to describe absorption properties of solvated systems. In particular, we focus on how to properly model the complexity of the solvation phenomenon, arising from dynamical aspects and specific, strong solute–solvent interactions. To this end, conformational and configurational sampling techniques, such as Molecular Dynamics, have to be coupled to accurate fully atomistic Quantum Mechanical/Molecular Mechanics (QM/MM) methodologies. By exploiting different illustrative applications, we show that an effective reproduction of experimental spectral signals can be achieved by delicately balancing exhaustive sampling, hydrogen bonding, mutual polarization, and nonelectrostatic effects.

Water Maintains the UV–Vis Spectral Features During the Insertion of Anionic Naproxen and Ibuprofen into Model Cell Membranes

“…MD runs coupled with QM/MM have been shown to be particularly reliable because, on the one hand, MD simulations allow the capture of the microscopic behavior of the molecules and have a good representation of macroscopic properties due to the ability of getting a set of different configurations of the equilibrated system, which on average represent the behavior of the system as a whole. − On the other hand, QM/MM approaches are relatively inexpensive because only a portion of the system to be excited is modeled at the QM level, while the environment is described classically with MM force fields. − By following such a computational strategy, it has been possible to get an outstanding reproduction of experimental UV–vis spectra at reduced computational cost. − There are different conceptual ways of combining QM and MM parts that differ from each other in the inclusion and method of describing the mutual interactions. ,, Possible approaches consist of mechanical and electrostatic embedding. In the latter, the interaction term is formulated in terms of a set of fixed charges, whereas more sophisticated approaches make charges to adjust to the QM density, , however, they may require specific parametrization for the considered environments .…”

UV-Resonance Raman Spectra of Systems in Complex Environments: A Multiscale Modeling Applied to Doxorubicin Intercalated into DNA

“…When the system under investigation is in solution, the complexity of modeling increases, and quantum mechanics/molecular mechanics (QM/MM) methods have been proven to be particularly successful, thanks to robust computational protocols developed in recent years. − Furthermore, in such systems, the partitioning between the QM and MM portions is generally straightforward because no covalent boundary exists between the solute and the solvent. Evidently, if the complexity of the system further increases (e.g., in the case of heterogeneous systems), the existing protocols for isolated or even solvated molecules must be adapted so that the new features can be properly considered in a physically consistent way.…”

Continuum vs. atomistic approaches to computational spectroscopy of solvated systems

“…13,41,[95][96][97] Differently, in polarizable QM/MM approaches nonequilibrium is automatically described by readjusting the MM polarizable quantities (multiples) to the transition density (we specify here the discussion to TDHF/TDDFT). 20,36,59,94,98 The other solvent degrees of freedom are kept frozen by fixing the geometry of the frames. Methods based on a combination of fixed multipoles and polarizable dipoles (such as Induced Dipoles and AMOEBA) have a clear correspondence with the implicit picture.…”

“…However, the static contribution can be seen as arising from differences in atomic electronegativities, while the polarization contribution is due to the chemical hardness, which enters the definition of the FQ matrix. 20,99 It is also clear that since in electrostatic embedding the MM multipolar distribution is fixed and does not depend on the QM density, neither nonequilibrium nor direct contributions to response equations are described, thus yielding an unrealistic picture of the electronic excitation. 20 As stated above, polarizable approaches introduce additional terms in the QM Hamiltonian.…”