arXiv subjects

Chiara Cappelli

Publications and source records attributed to Chiara Cappelli.

At least 19 recordsLinked to original sources

arXiv2026

Strong light-matter interactions in hybrid polaritonic systems

Strong light-matter coupling gives rise to polaritons - hybrid excitations whose mixed photonic and matter character enables control over optical, electronic and chemical properties. This Feature Article surveys the main architectures supporting polariton formation, including photonic microcavities, plasmonic nanostructures, open cavities and metasurfaces, and outlines how inorganic semiconductors, organic aggregates and hybrid systems access strong and ultrastrong coupling. Key phenomena such as coherent dynamics, vibronic interactions, dark-state reservoirs and polariton-mediated energy and electron transport are discussed, together with the experimental and theoretical tools used to study them. We highlight examples where strong coupling modifies charge transport, energy flow and chemical reactivity, and we summarize emerging regimes, including intermediate and dark-strong coupling, that broaden the landscape of hybrid light-matter physics.

physics.optics↗

arXiv2026

DMRG/FQ: a Polarizable Embedding Approach Combining Density Matrix Renormalization Group and Fluctuating Charges

We present an integrated multiscale framework that combines the Density Matrix Renormalization Group (DMRG) with a polarizable fluctuating-charge (FQ) force field for the simulation of electronic excited states in solution. The method exploits the capabilities of DMRG to accurately describe systems with strong static correlation, while the FQ model provides a self-consistent and physically grounded representation of solvent polarization within a QM/MM embedding. The DMRG/FQ approach is applied to representative solvated systems, using extensive molecular dynamics sampling. The method yields reliable excitation energies, solvatochromic shifts, and a close agreement with available experimental data. The results highlight the importance of mutual polarization for capturing specific solute-solvent interactions, particularly in systems where hydrogen bonding or directional interactions play a dominant role.

physics.chem-ph↗

arXiv2025

Analytical Nuclear Gradients for the Multiconfigurational Self-Consistent Field Method Coupled with the Polarizable Fluctuating Charges Model

The multiscale model combining the multiconfigurational self-consistent field (MCSCF) method with the fully atomistic polarizable Fluctuating Charges (FQ) force field (J. Chem. Theory Comput. 2024, 20, 9954-9967) is here extended to the calculation of analytical nuclear gradients. The gradients are derived from first principles, implemented in the OpenMolcas package, and validated against numerical references. The resulting MCSCF/FQ nuclear gradients are employed to simulate vibronic absorption spectra of aromatic molecules in aqueous solution, namely benzene and phenol. By integrating this approach with molecular dynamics simulations, both solute conformational flexibility and the dynamical aspects of solvation are properly captured. The computed spectra reproduce experimental profiles and relative band intensities with remarkable accuracy, demonstrating the capability of the MCSCF/FQ model to simultaneously describe the multireference character of the solute and its interaction with the solvent environment.

physics.chem-ph↗

arXiv2025

Vertical Excitation Energies of Embedded Systems: The Vertical Excitation Model (VEM) within Polarizable QM/MM

Polarizable Quantum Mechanics/Molecular Mechanics (QM/MM) approaches based on fluctuating charges and dipoles (QM/FQ(F$\mu$)) are formulated within the state-specific Vertical Excitation Model (VEM) to compute vertical excitation energies of solvated systems. This methodology overcomes the limitations of the widely used Linear Response (LR) approach. While LR can capture the dynamic response of the solvent to the QM transition density, it neglects the solvent reorganization that follows solute relaxation upon electronic excitation. In contrast, the VEM framework explicitly accounts for this effect. Benchmark calculations of vertical excitation energies using QM/FQ(F$\mu$) are reported for a representative set of solutes - acrolein, acetone, caffeine, p-nitroaniline, coumarin 153, doxorubicin, and betaine-30 - comparing VEM with LR, corrected LR (cLR), and cLR 2 schemes. The results reveal notable variations in solvent response depending on the character of the electronic transition and demonstrate that optimal accuracy can be achieved by selecting the most appropriate model for each specific system and excitation.

physics.chem-ph↗

arXiv2025

Atomistic QM/Classical Modeling of Surface-Enhanced Infrared Absorption

We present a multiscale quantum mechanics/classical (QM/MM) approach for modeling surface-enhanced infrared absorption (SEIRA) spectra of molecules adsorbed on plasmonic nanostructures. The molecular subsystem is described at the density functional theory (DFT) level, while the plasmonic material is represented using fully atomistic, frequency-dependent Fluctuating Charges ($\omega$FQ) and Fluctuating Charges and Dipoles ($\omega$FQF$\mu$) models. These schemes enable an accurate and computationally efficient description of the plasmonic response of both graphene-based materials and noble metal nanostructures, achieving accuracy comparable to ab initio methods. The proposed methodology is applied to the calculation of SEIRA spectra of adenine adsorbed on gold nanoparticles and graphene sheets. The quality and robustness of the approach are assessed through comparison with surface-enhanced Raman scattering (SERS) spectra and available experimental data. The results demonstrate that the proposed framework provides a reliable route to simulate vibrational responses of plasmon-molecule hybrid systems.

physics.chem-ph↗

arXiv2025

eT 2.0: An efficient open-source molecular electronic structure program

The eT program is an open-source electronic structure program with emphasis on performance and modularity. As its name suggests, the program features extensive coupled cluster capabilities, performing well compared to other electronic structure programs, and, in some cases, outperforming commercial alternatives. However, eT is more than a coupled cluster program; other models based on wave function theory (such as full and reduced space configuration interaction and a variety of self-consistent field models) and density functional theory are supported. The second major release of the program, eT 2.0, has specialized functionality for strong light-matter coupling conditions. In addition, it includes a wide range of optimizations and algorithmic improvements, as well as new capabilities for exploring potential energy surfaces and for modeling experiments in the UV and X-ray regimes. Molecular gradients are now available at the coupled cluster level, and high-accuracy spectroscopic simulations are available at reduced computational cost within the multilevel coupled cluster and multiscale frameworks. We present the modifications to the program since its first major release, eT 1.0, highlighting some notable new features and demonstrating the performance of the new version relative to the first release and to other established electronic structure programs.

physics.chem-ph↗

arXiv2025

plasmonX: an Open-Source Code for Nanoplasmonics

We present the first public release of plasmonX, a novel open-source code for simulating the plasmonic response of complex nanostructures. The code supports both fully atomistic and implicit descriptions of nanomaterials. In particular, it employs the frequency-dependent fluctuating charges ($\omega$FQ) and dipoles ($\omega$FQF$\mu$) models to describe the response properties of atomistic structures, including simple and $d$-metals, graphene-based structures, and multi-metal nanostructures. For implicit representations, the Boundary Element Method is implemented in both the dielectric polarizable continuum model (DPCM) and integral equation formalism (IEF-PCM) variants. The distribution also includes a post-processing module that enables analysis of electric field-induced properties such as charge density and electric field patterns.

cond-mat.mes-hall↗

arXiv2025

Quantum Dynamics of Dissipative Polarizable Media

Classical polarizable approaches have become the gold standard for simulating complex systems and processes in the condensed phase. These methods describe intrinsically dissipative polarizable media, requiring a formal definition within the framework of open quantum systems. We present a Hamiltonian formulation for the quantum dynamics of polarizable sources based on a generalized theory of the damped harmonic oscillator, using pseudo-boson theory to characterize their coherent state dynamics. We then apply our theory to the study of the optical response of two plasmonic systems. Furthermore, by exploiting the phase space formulation of quantum mechanics and the integrability of quadratic Hamiltonians, we derive a self-consistent relation for the emitted electric field of the polarizable medium under the semiclassical approximation, based on exact formulas for medium polarization. Finally, we derive the master equation describing the open dynamics of a quantum system interacting with the quantum polarizable medium, along with analytical expressions for correlation functions calculated over arbitrary Gaussian states.

quant-ph↗

arXiv2024

Mixed Atomistic-Implicit Quantum/Classical Approach to Molecular Nanoplasmonics

A multiscale QM/classical approach is presented, that is able to model the optical properties of complex nanostructures composed of a molecular system adsorbed on metal nanoparticles. The latter are described by a combined atomistic-continuum model, where the core is described using the implicit boundary element method (BEM) and the surface retains a fully atomistic picture and is treated employing the frequency-dependent fluctuating charge and fluctuating dipole ($ω$FQF$μ$) approach. The integrated QM/$ω$FQF$μ$-BEM model is numerically compared with state-of-the-art fully atomistic approaches, and the quality of the continuum/core partition is evaluated. The method is then extended to compute Surface-Enhanced Raman Scattering (SERS) within a Time-Dependent Density Functional Theory (TDDFT) framework.

physics.chem-ph↗

arXiv2024

Fully Polarizable Multiconfigurational Self-consistent Field/Fluctuating Charge Approach

A multiscale model based on the coupling of the multiconfigurational self-consistent field (MCSCF) method and the classical atomistic polarizable Fluctuating Charges (FQ) force field is presented. The resulting MCSCF/FQ approach is validated by exploiting the CASSCF scheme through application to compute vertical excitation energies of formaldehyde and para-nitroaniline in aqueous solution. The procedure is integrated with molecular dynamics simulations to capture the solute's conformational changes and the dynamic aspects of solvation. Comparative analysis with alternative solvent models, gas-phase calculations, and experimental data provides insights into the model's accuracy in reproducing solute-solvent molecular interactions and spectral signals.

physics.chem-ph↗

arXiv2024

Atomistic Multiscale Modeling of Colloidal Plasmonic Nanoparticles

A novel fully atomistic multiscale classical approach to model the optical response of solvated real-size plasmonic nanoparticles (NPs) is presented. The model is based on the coupling of the Frequency Dependent Fluctuating Charges and Fluctuating Dipoles ($ω$FQF$μ$), specifically designed to describe plasmonic substrates, and the polarizable Fluctuating Charges (FQ) classical force field to model the solvating environment. The resulting $ω$FQF$μ$/FQ approach accounts for the interactions between the radiation and the NP, as well as with the surrounding solvent molecules, by incorporating mutual interactions between the plasmonic substrate and solvent. $ω$FQF$μ$/FQ is validated against reference TD-DFTB/FQ calculations, demonstrating remarkable accuracy, particularly in reproducing plasmon resonance frequency shifts for structures below the quantum-size limit. The flexibility and reliability of the approach are also demonstrated by simulating the optical response of homogeneous and bimetallic NPs dissolved in pure solvents and solvent mixtures.

cond-mat.mes-hall↗

arXiv2024

Real-Time Formulation of Atomistic Electromagnetic Models for Plasmonics

Investigating nanoplasmonics using time-dependent approaches permits shedding light on the dynamic optical properties of plasmonic structures, which are intrinsically connected with their potential applications in photochemistry and photoreactivity. This work proposes a real-time extension of our recently developed fully atomistic approaches $ω$FQ and $ω$FQF$μ$. These methods successfully reproduce quantum size effects in metal nanoparticles, including plasmon shifts for both simple and $d$-metals, even below the quantum size limit. Also, thanks to their atomistic nature and the phenomenological inclusion of quantum tunneling effects, they can effectively describe the optical response of subnanometer junctions. By incorporating real-time dynamics, the approach provides an efficient framework for studying the time-dependent optical behavior of metal nanostructures, including the decoherence of plasmon excitations.

cond-mat.mes-hall↗

arXiv2022

Assessing the Quality of QM/MM Approaches to Describe Vacuo-to-water Solvatochromic Shifts

The performance of different Quantum Mechanics/Molecular Mechanics embedding models to compute vacuo-to-water solvatochromic shifts are investigated. In particular, both non-polarizable and polarizable approaches are analyzed and computed results as compared to reference experimental data. We show that none of the approaches outperforms the others and that errors strongly depend on the nature of the molecular transition. Thus, we prove that the best choice of embedding model highly depends on the molecular system, and that the use a specific approach as a black-box can lead to significant errors and sometimes totally wrong predictions.

physics.chem-ph↗

arXiv2021

Fully Atomistic Modeling of Realistic Plasmonic Materials: Assessing the Performance of Iterative Solvers

The fully atomistic modeling of real-size plasmonic nanostructures is computationally demanding, therefore most calculations are limited to small-to-medium sized systems. However, plasmonic properties strongly depend on the actual shape and size of the samples. In this paper we substantially extend the applicability of classical, fully atomistic approaches by exploiting state-of-the-art numerical iterative Krylov-based techniques. In particular, we focus on the recently developed $ω$FQ model, when specified to carbon nanotubes, graphene-based nanostructures and metal nanoparticles. The performance of Generalized Minimal Residual (GMRES) and Quasi-Minimum Residual (QMR) algorithms is studied, with special emphasis on the dependence of the convergence rate on the dimension of the structures (up to 1 million atoms) and the physical parameters entering the definition of the atomistic approach.

physics.comp-ph↗

arXiv2020

Multilevel Density Functional Theory

We introduce a novel density-based multilevel approach in density functional theory. In this multilevel density functional theory (MLDFT), the system is partitioned in an active and an inactive fragment, and all interactions are retained between the two parts. In MLDFT, the Kohn-Sham equations are solved in the MO basis for the active part only, while keeping the inactive density frozen. This results in a reduction of computational cost. We outline the theory and implementation, and discuss applications to aqueous solutions of methyloxirane and glycidol.

physics.chem-ph↗

arXiv2020

Graphene Plasmonics: a Novel Fully Atomistic Approach for Realistic Structures

We demonstrate that the plasmonic properties of realistic graphene and graphene-based materials can effectively and accurately be modeled by a novel, fully atomistic, yet classical, approach, named $ω$FQ. Such model is able to reproduce all plasmonic features of these materials, and their dependence on shape, dimension and fundamental physical parameters (Fermi energy, relaxation time and two-dimensional electron density). Remarkably, $ω$FQ is able to accurately reproduce experimental data for realistic structures of hundreds of nanometers ($\sim$ 370.000 atoms), which cannot be afforded by any \emph{ab-initio} method. Also, the atomistic nature of $ω$FQ permits the investigation of complex shapes, which can hardly be dealt with by exploiting widespread continuum approaches.

cond-mat.mes-hall↗

arXiv2019

Electronic Transitions for a Fully Polarizable QM/MM Approach Based on Fluctuating Charges and Fluctuating Dipoles: Linear and Corrected Linear Response Regimes

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 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.

physics.chem-ph↗

arXiv2019

On the Calculation of IR Spectra with a Fully Polarizable QM/MM Approach Based on Fluctuating Charges and Fluctuating Dipoles

The 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 evaluation of nuclear gradients and the calculation of IR spectra of molecular systems in condensed phase. To this end, analytical equations defining first and second energy derivatives with respect to nuclear coordinates are derived and discussed. The potentialities of the approach are shown by applying the model to the calculation of IR spectra of Methlyoxirane, Glycidol and Gallic Acid in aqueous solution. The results are compared with the continuum QM/PCM and the polarizable QM/FQ, which is based on Fluctuating Charges only.

physics.chem-ph↗