Manuel A. Aguilar

Nonequilibrium solvation: An ab initio quantum‐mechanical method in the continuum cavity model approximation

The electrostatic relationships necessary for the quantum‐mechanical evaluation of the properties of a solute experiencing sudden changes in its internal charge distribution are here presented in a form suitable to perform accurate quantum‐mechanical calculations of the solute properties. Attention has been paid to express the boundary conditions in the most convenient form and to avoid further constraints on the elaboration of the computational procedures. The approach exploits the separation of orientational (inertial) and electronic (inertialess) components of the polarization and complements the polarizable continuum method [Chem. Phys. 65, 239 (1982)], usually employed for static descriptions. Examples of application of the method to photoionization and electronic transitions processes are shown.

Study of solvent effects by means of averaged solvent electrostatic potentials obtained from molecular dynamics data

We present the theory and implementation of a new approach for studying solvent effects. The electronic structure of the solute, calculated at the ab initio level, is obtained in the presence of the surrounding medium. We employ a mean field theory in which the solvent response is described by means of point charges chosen in such a way that they reproduce the average value of the solvent electrostatic potential calculated from molecular dynamics data. In this way, the complete solvent potential can be introduced into the solute Hamiltonian without making use of a one‐center multiple expansion of the solute‐solvent potential. In the proposed method, only one quantum calculation has to be performed and a great number of configurations can easily be included making the calculation statistically significant. We show that, despite the large fluctuations in the solute charge distribution induced by the solvent, the proposed mean field theory adequately reproduces the energetics and properties of formamide and water molecules in aqueous solution.

A multiconfiguration self-consistent field/molecular dynamics study of the (n→π*)1 transition of carbonyl compounds in liquid water

A model is presented for the electrostatic component of solvatochromic shifts in vertical electronic excitation energies. The model, which makes use of the mean-field approximation, combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. The method is implemented at the multiconfigurational self-consistent field level. We present illustrative applications to the (n→π*)1 transitions of formaldehyde, acetaldehyde, and acetone in liquid water. The solvent shifts obtained compare well with other ab initio QM/MM calculations and when the electron correlation components are included with the experimental solvent shift, but differ from the results obtained with semiempirical QM/MM and continuum models.

Geometry optimization of molecules in solution: Joint use of the mean field approximation and the free-energy gradient method

The average solvent electrostatic potential/molecular dynamics (ASEP/MD) and the free-energy gradient methods are applied together with the multidimensional geometry optimization of molecules in solution. The systems studied were formamide in aqueous solution and water and methanol in liquid phase. The solute molecules were described through ab initio quantum mechanics methods (density dunctional theory or Moller–Plesset second order perturbation theory) while the solvent structure was obtained from Molecular Dynamics calculations. The method is very efficient; the increase in computation time is minimal with respect to previous ASEP/MD versions that worked at a fixed geometry. Despite the use of the mean field approximation in the calculation of the solvent reaction potential the agreement with previous theoretical calculations was satisfactory. Large changes were observed in the solute charge distribution induced by the solvent, and the solute polarization was accompanied by an increase in the solvent s...

Solute-solvent interactions. a simple procedure for constructing the solvent cavity for retaining a molecular solute

Abstract The objective of this work is to propose a procedure for the adaptation of the molecular solute cavity size in a solvent, when we use the SCF LCAO MO treatment of solute-solvent interactions, following the continuum model. The atoms studied are H, Li, Be, C, N, O, F, NA, Mg, Si, P, and Cl. For all of them we find a functional dependence of the radii on the net charge considering the effect of electronic cloud and the basis set. The obtained formulae were used to compute the internal and electrostatic free energy changes in solvation processes, using the Tomasi model.

Electron correlation and solvation effects. II. The description of the vibrational properties of a water molecule in a dielectric given by the application of the polarizable continuum model with inclusion of correlation effects

Abstract The ab initio quantum mechanical version of the polarizable continuum model has recently been improved by introducing electron correlation effects (paper I). In the present paper we continue the analysis of the performances of this method by examining the potential energy surface of H 2 O in vacuo and in a medium (ϵ=78.3), at the SCF, MBP2 and MBP3 levels of the formalism. A short discussion about the relevance of this approach to gain some information about structural features of bulk water is also included.

Solvent effects by means of averaged solvent electrostatic potentials: Coupled method

In this article we propose a mean field theory that permits the calculation of solvent effects in a direct way by combining quantum mechanics and molecular dynamics simulations. Because of the reduced number of necessary quantum calculations, it is possible to get the same level of theory used for molecules in vacuo. The electronic structure of the solute in solution and the solvent structure around it are optimized in a self‐consistent way. The main characteristics of the proposed method are high‐level quantum calculations in the representation of the solute, a detailed description of the solvent structure through molecular dynamics calculation, inclusion of the mutual polarization of the solute and solvent molecules, and an accurate description of the solute–solvent interaction energy. As an application of the model we studied the polarization of quantum mechanically treated water and methanol molecules in the liquid phase.

A new method to locate saddle points for reactions in solution by using the free‐energy gradient method and the mean field approximation

A new method for calculating saddle points of reactions in solution is presented. The main characteristics of the method are: (1) the solute–solvent system is described by the averaged solvent electrostatic potential/molecular dynamics method (ASEP/MD). This is a quantum mechanics/molecular mechanics method (QM/MM) that makes use of the mean field approximation (MFA) and that permits one to simultaneously optimize the electronic structure and geometry of the solute molecule and the solvent structure around it. (2) The transition state is located by the joint use of the free‐energy gradient method and the mean field approximation. An application to the study of the Menshutkin reaction between NH3 and CH3Cl in aqueous solution is discussed. The accuracy and usefulness of the proposed method is checked through comparison with other methods.

A theoretical study of liquid alcohols using averaged solvent electrostatic potentials obtained from molecular dynamics simulations: Methanol, ethanol and propanol

We applied a quantum mechanics/molecular mechanics method that makes use of the mean field approximation to study the polarization of several alcohols in the liquid phase. The method is based on the calculation of the averaged solvent electrostatic potential from molecular dynamics data. Because of the reduced number of quantum calculations that our approximation involves, it permits the use of flexible basis sets, the consideration of the electron correlation and the solvent and solute polarization. We found that the molecules studied undergo strong polarization when they pass from the gas to the liquid phase. From this point of view, the polarization methanol displays a behavior lightly different from ethanol and propanol. The vaporization energies are very well reproduced especially when the correlation energy is included. The differences with the experimental values are less than 3% in the three systems studied. Finally, we consider the effect on the thermodynamics and the structure of the solution of...

Polarizable continuum model calculations including electron correlation in the ab initio wavefunction

Abstract Some calculations are presented using a recently developed version of the ab initio polarizable continuum model that includes electron correlation in the representation of the solute. Calculations were carried out to estimate the self-consistent field electronic solvation energies and one-electron properties of several molecules. Comparison is made of results at several levels of approximation.