Chiara Cappelli

Linear response theory and electronic transition energies for a fully polarizable QM/Classical Hamiltonian

A fully polarizable quantum/classical Hamiltonian including SCF (HF or DFT), fluctuating charge, and polarizable continuum regions is introduced and implemented for electronic energies of ground and excited states, using, in the latter case, a linear response formulation. After calibration and validation of the approach, preliminary results are presented for pyrimidine in aqueous solution and for retinal in a rhodopsin mimic. The results are consistent with more tested methodologies and pave the route toward fully consistent yet effective simulations of large systems of technological and/or biological interest in their natural environments.

Integrated computational approaches for spectroscopic studies of molecular systems in the gas phase and in solution: pyrimidine as a test case

An integrated computational approach built on quantum mechanical (QM) methods, purposely tailored inter- and intra-molecular force fields and continuum solvent models combined with time-independent and time-dependent schemes to account for nuclear motion effects is applied to the spectroscopic investigation of pyrimidine in the gas phase as well as in aqueous and CCl4 solutions. Accurate post-Hartree–Fock methodologies are employed to compute molecular structure, harmonic vibrational frequencies, energies and oscillator strengths for electronic transitions in order to validate the accuracy of approaches rooted into density functional theory with emphasis also on hybrid QM/QM′ models. Within the time-independent approaches, IR spectra are computed including anharmonicities through perturbative corrections while UV–vis line-shapes are simulated accounting for the vibrational structure; in both cases, the environmental effects are described by continuum models. The effects of conformational flexibility, including solvent dynamics, are described through time-dependent models based on purposely DFT-tailored force fields applied to molecular dynamics simulations and on QM computations of spectroscopic properties. Such procedures are exploited to simulate IR and UV–vis spectra of pyrimidine in the gas phase and in solutions, leading in all cases to good agreement with experimental observations and allowing to dissect different effects underlying spectral phenomena.

Anharmonic theoretical simulations of infrared spectra of halogenated organic compounds

The recent implementation of the computation of infrared (IR) intensities beyond the double-harmonic approximation [J. Bloino and V. Barone, J. Chem. Phys. 136, 124108 (2012)] paved the route to routine calculations of infrared spectra for a wide set of molecular systems. Halogenated organic compounds represent an interesting class of molecules, from both an atmospheric and computational point of view, due to the peculiar chemical features related to the halogen atoms. In this work, we simulate the IR spectra of eight halogenated molecules (CH2F2, CHBrF2, CH2DBr, CF3Br, CH2CHF, CF2CFCl, cis-CHFCHBr, cis-CHFCHI), using two common hybrid and double-hybrid density functionals in conjunction with both double- and triple-ζ quality basis sets (SNSD and cc-pVTZ) as well as employing the coupled-cluster theory with basis sets of at least triple-ζ quality. Finally, we compare our results with available experimental spectra, with the aim of checking the accuracy and the performances of the computational approaches.

Nonequilibrium formulation of infrared frequencies and intensities in solution: Analytical evaluation within the polarizable continuum model

We present a nonequilibrium approach to the analytical evaluation of infrared frequencies and intensities for molecules in solution within the polarizable continuum model framework. Vibrational frequencies and intensities are obtained in the harmonic approximation as the second derivatives of the suitable nonequilibrium free energy. A numerical application to the calculation of carbonyl stretching frequencies and intensities of a series of ketones at the density functional level is shown. In order to effectively compare theoretical and experimental data the coupling between the solvent and the probing field is also taken into account. The dependence of the results on the choice of the functional and of the basis set, as well as on the cavity geometry, is considered.

On the Calculation of Vibrational Frequencies for Molecules in Solution Beyond the Harmonic Approximation

We report some results on the calculation of vibrational spectra of molecules in condensed phase with accounting simultaneously for anharmonicity and solute-solvent interactions, the latter being described by means of the polarizable continuum model (PCM). Density functional theory force fields are employed as well as a new implementation of the PCM cavity and its derivatives. The results obtained for formaldehyde and simple peptide prototypes show that our approach is able to yield a quantitative agreement with experiments for vacuo-to-solvent harmonic and anharmonic frequency shifts.

Analytical First and Second Derivatives for a Fully Polarizable QM/Classical Hamiltonian

In this work, we present the derivation and implementation of analytical first and second derivatives for a fully polarizable QM/MM/PCM energy functional. First derivatives with respect to both QM- and MM-described nuclear coordinates and electric perturbations are derived and implemented, and some preliminary application is shown. Analytical second derivatives with respect to nuclear and electric perturbations are then derived, and some numerical test is presented both for a solvated system and for a cromophore embedded in a biological matrix.

Effective Time-Independent Calculations of Vibrational Resonance Raman Spectra of Isolated and Solvated Molecules Including Duschinsky and Herzberg-Teller Effects

We present a method of modeling vibrational resonance Raman scattering (RRS) spectra of isolated and solvated systems with the inclusion of Franck-Condon (FC) and Herzberg-Teller (HT) effects and a full account for possible differences between the harmonic potential energy surfaces of the initial and resonant electronic states. It describes fundamentals, overtones, and combination bands and computes the RRS spectrum as a two-dimensional function of the incident and scattered frequencies. The theoretical foundations of the method are described and the differences with other currently available methodologies are outlined. Applications to the phenoxyl radical in the gas phase and indolinedimethine-malononitrile (IDMN) in acetonitrile and cyclohexane solution are reported, as well as comparisons with available experimental data.

The Optical Rotation of Methyloxirane in Aqueous Solution: A Never Ending Story?

The long-standing problem of the calculation of the optical rotation (OR) of (R)-methyloxirane in aqueous solution at different wavelengths is solved by means of a novel gauge-invariant computational protocol able to take into account at the same time for intramolecular averaging specific and bulk solvent effects, leading for the first time to a quantitative agreement (both sign and absolute value) between computed and experimental OR values at several frequencies.

Toward an Accurate Modeling of Optical Rotation for Solvated Systems: Anharmonic Vibrational Contributions Coupled to the Polarizable Continuum Model

We present a newly implemented methodology to evaluate vibrational contributions (harmonic and anharmonic) to the optical rotation of solvated systems described by means of the polarizable continuum model (PCM). Proper account of an incomplete solvation regime in the treatment of both the electronic property and the molecular vibrations is considered, as well as the inclusion of cavity field effects. In order to assess the quality of our approach, test calculations on (R)-methyloxirane in various solvents and (S)-N-acetylproline amide in cyclohexane and aqueous solution are presented. The comparison with experimental findings is also shown.

Towards an accurate description of anharmonic infrared spectra in solution within the polarizable continuum model: Reaction field, cavity field and nonequilibrium effects

We present a newly developed and implemented methodology to perturbatively evaluate anharmonic vibrational frequencies and infrared (IR) intensities of solvated systems described by means of the polarizable continuum model (PCM). The essential aspects of the theoretical model and of the implementation are described and some numerical tests are shown, with special emphasis towards the evaluation of IR intensities, for which the quality of the present method is compared to other methodologies widely used in the literature. Proper account of an incomplete solvation regime in the treatment of the molecular vibration is also considered, as well as inclusion of the coupling between the solvent and the probing field (cavity field effects). In order to assess the quality of our approach, comparison with experimental findings is reported for selected cases.