Electronic transitions for a fully polarizable QM/MM approach based on fluctuating charges and fluctuating dipoles: Linear and corrected linear response regimes

Abstract

Fully polarizable QM/MM approach based on fluctuating charges and fluctuating dipoles, named QM/FQFµ (J. Chem. Theory Comput. 2019, 15 2233-2245), is extended to the calculation of vertical excitation energies of solvated molecular systems. Excitation energies are defined within two different solvation regimes, i.e. linear response (LR), where the response of the MM portion is adjusted to the QM transition density, and corrected-Linear Response (cLR) in which the MM response is adjusted to the relaxed QM density, thus being able to account for charge equilibration in the excited state. The model, which is specified in terms of three physical parameters (electronegativity, chemical hardness, and polarizability) is applied to vacuo-to-water 1 arXiv:1906.03852v1 [physics.chem-ph] 10 Jun 2019 solvatochromic shifts of aqueous solutions of para-nitroaniline, pyridine and pyrimidine. The results show a good agreement with their experimental counterparts, thus highlighting the potentialities of this approach. IntroductionExcited-state phenomena play a crucial role in many application fields, as for instance photocatalysis, optical information storage and solar cells. In the past decades, theoretical modeling of excited-state properties of molecules in the gas-phase has become a widespread strategy of investigation, 1 giving precious information on, for instance, the nature of the electronic excitation, 2-4 nuclei-electron coupling effects 5-7 and excited state electron dynamics. [8][9][10] However, for electronic phenomena taking place in the condensed phase, 11-19 the interplay between the molecule and its environment can substantially alter the electronic response to external electromagnetic fields. Therefore, any accurate modeling of excited states of solvated systems asks for reliable theoretical approaches to include the effects of the environment at all levels of the excitation phenomenon.Most of the currently available approaches to describe the effects of the external environment on molecular properties belong to the class of the so-called focused models; 20-25 the attention is focused on the molecule and the environment is treated a lower level of sophistication as it modifies, but not determines, the molecular response to the external radiation. In order to keep the atomistic description of the environment, thus substantially overcoming well-known and amply used continuum solvent descriptions, 23,25-29 multiscale QM/Molecular Mechanics (MM) approaches have been developed. 30,31 In such models, the molecule (solute) is treated at the QM level, whereas the environment (solvent) is modelled by means of classical MM force fields (FF). The interaction between the QM and MM portions is usually described in terms of electrostatic forces, although approaches to include non-electrostatic QM/MM interactions have been proposed recently. [32][33][34] In order to fully capure the physics of solute-

Molecular spectroscopy of aqueous solutions: a theoretical perspective

“…To show that, in Fig. 5 computed UV-vis absorption spectra of p-nitroaniline and pyridine in aqueous solution are reported; 19 there, QM/FQ and QM/FQFm are compared. The figure shows how polarization affects the general shape of the spectra as well as solvatochromic shifts computed with respect to the vacuum.…”

e T 1.0: An open source electronic structure program with emphasis on coupled cluster and multilevel methods

“…87 For QM/PCM, we use two different atomic radii, UFF 85 (QM/PCM c ) and Bondi 86 (QM/PCM d ), and the dielectric permittivity of water was set to ε = 78.39. For QM/MM, 64 snapshots were extracted from a classical molecular dynamics simulation 88 [see Fig. 7(a) for an example structure].…”

“…This is probably due to the larger molecular dipole moments of the water molecules in this parameterization. 88,89 In Fig. 7(c), the convoluted spectra calculated using QM/PCM c and QM/PCM d (top), and QM/FQ a and QM/FQ b (bottom), are presented with their vacuum counterparts.…”

Development and application of quantum mechanics/molecular mechanics methods with advanced polarizable potentials

“…For the MM environment many QM/MM simulations employ nonpolarizable point‐charge force fields (npFFs) 21–23 . More recently, increase in computational speed and algorithmic developments have allowed the use of more accurate potentials which may include explicit representation of electronic polarization, more elaborate electrostatics, and in some cases, explicit inclusion of other quantum effects 24–27 …”

“…[21][22][23] More recently, increase in computational speed and algorithmic developments have allowed the use of more accurate potentials which may include explicit representation of electronic polarization, more elaborate electrostatics, and in some cases, explicit inclusion of other quantum effects. [24][25][26][27] The coupling of QM and MM methods to represent a system gives rise to several challenges such as how to couple these two levels of theory across covalent boundaries, how to treat long-range effects in the context of QM subsystems, coupling of the quantum and classical Hamiltonians and how to treat the explicit couplings, among others. In this review, we present approaches that tackle several of these issues.…”