Benedetta Mennucci

A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anisotropic dielectrics

We present a new integral equation formulation of the polarizable continuum model (PCM) which allows one to treat in a single approach dielectrics of different nature: standard isotropic liquids, intrinsically anisotropic medialike liquid crystals and solid matrices, or ionic solutions. The present work shows that integral equation methods may be used with success also for the latter cases, which are usually studied with three-dimensional methods, by far less competitive in terms of computational effort. We present the theoretical bases which underlie the method and some numerical tests which show both a complete equivalence with standard PCM versions for isotropic solvents, and a good efficiency for calculations with anisotropic dielectrics.

Continuum solvation models: A new approach to the problem of solute’s charge distribution and cavity boundaries

In continuum solvation models the definition of a cavity that embeds the solute molecule leads to problems related to the portion of solute’s electronic charge lying outside its boundaries (charge tails). The correction strategies developed so far can be shown to work insufficiently, since they only correct the global charge defect, but lead to considerable local errors. The present paper will be focused on the theoretical and technical aspects of this problem, and it will present in detail a new method which allows a very refined treatment of solute’s charge tails in the outer space; some numerical results of solutes in water will be shown and discussed. As further analyses, the introduction of Pauli repulsion term will be considered, and the implications all these effects have on molecular properties, such as (hyper)polarizabilities, numerically evaluated. The new approach has been implemented within the framework of the polarizable continuum model (PCM).

The IEF version of the PCM solvation method: an overview of a new method addressed to study molecular solutes at the QM ab initio level

Abstract The integral equation formalism (IEF) is a recent method (the grounds have been elaborated at the beginning of 1997) addressed to solve the electrostatic solvation problem at the QM level with the aid of apparent surface charges (ASC). IEF uses a new formalism of this problem, based on integral operators never used before in the chemical community and it manages to treat on the same footing linear isotropic solvent models, as well as anisotropic liquid crystals and ionic solutions. In this overview we emphasize the good performances of IEF at the lowest level of its potentialities, i.e. for isotropic solvents, as a new approach to compute solvation free energies and properties (dipole hyperpolarizabilities) of molecular solutes, as well as energy gradients for geometry optimization procedures. Finally we present a new IEF implementation of the nonequilibrium problem for electronic spectra which appears to be decidedly competitive with the previous more standard ASC formulations.

Ab initio study of ionic solutions by a polarizable continuum dielectric model

Abstract A new implementation of a recently developed formalism to describe chemical systems in ionic solutions is presented. It allows ab initio calculations at the Hartree–Fock and density functional levels on closed and open shell systems, taking into account the ionic atmosphere effects at not too large concentrations. Test calculations on simple systems are compared to experimental data and to values obtained by numerical integration of the Poisson–Boltzmann equation. A more complex system, namely the glycine radical in aqueous solution, is also analyzed.

Geometries and properties of excited states in the gas phase and in solution: Theory and application of a time-dependent density functional theory polarizable continuum model

In this paper we present the theory and implementation of analytic derivatives of time-dependent density functional theory (TDDFT) excited states energies, both in vacuo and including solvent effects by means of the polarizable continuum model. The method is applied to two case studies: p-nitroaniline and 4-(dimethyl)aminobenzonitrile. For both molecules PCM-TDDFT is shown to be successful in supporting the analysis of experimental data with useful insights for a better understanding of photophysical and photochemical pathways in solution.

Definition of the hydrogen bond (IUPAC Recommendations 2011)

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

New applications of integral equations methods for solvation continuum models: ionic solutions and liquid crystals

We present a new method for solving numerically the equations associated with solvation continuum models, which also works when the solvent is an anisotropic dielectric or an ionic solution. This method is based on the integral equation formalism. Its theoretical background is set up and some numerical results for simple systems are given. This method is much more effective than three‐dimensional methods used so far, like finite elements or finite differences, in terms of both numerical accuracy and computational costs.

Formation and relaxation of excited states in solution : A new time dependent polarizable continuum model based on time dependent density functional theory

In this paper a novel approach to study the formation and relaxation of excited states in solution is presented within the integral equation formalism version of the polarizable continuum model. Such an approach uses the excited state relaxed density matrix to correct the time dependent density functional theory excitation energies and it introduces a state-specific solvent response, which can be further generalized within a time dependent formalism. This generalization is based on the use of a complex dielectric permittivity as a function of the frequency, epsilonomega. The approach is here presented in its theoretical formulation and applied to the various steps involved in the formation and relaxation of electronic excited states in solvated molecules. In particular, vertical excitations (and emissions), as well as time dependent Stokes shift and complete relaxation from vertical excited states back to ground state, can be obtained as different applications of the same theory. Numerical results on two molecular systems are reported to better illustrate the features of the model.

Recent Advances in the Description of Solvent Effects with the Polarizable Continuum Model

Publisher Summary This chapter presents a set of methods addressed to the study of solvation problems at the quantum mechanical level with the use of polarizable continuum model (PCM). The chapter describes the evolution of continuous methods since the first proposals or the first formulation at a QM level, semiempirical, or ab initio. PCM belongs to the family of methods in which focuses on a limited portion of matter, the ‘solute’ (one or more molecules) while the remaining, and larger, portion of the solution, called here the ‘solvent’, is treated at a lower level of accuracy. PCM is a quantum mechanical (QM) method in which use is made of an effective Hamiltonian for the solute M , and the corresponding Schrodinger equation is generally (but not compulsory) treated at the ab initio level. PCM makes use of continuous solvent distributions to describe the solute-solvent interaction potential. The chapter demonstrates that PCM manages to treat not only the well known model based on a uniform isotropic dielectric description of the solvent, whose interactions are limited to the electrostatic terms, but also more complex models including interactions of different physical origin and other solvent distribution functions.