M. L. Sanchez

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...

ASEP/MD: A program for the calculation of solvent effects combining QM/MM methods and the mean field approximation ☆

ASEP/MD is a computer program designed to implement the Averaged Solvent Electrostatic Potential/Molecular Dynamics (ASEP/MD) method developed by our group. It can be used for the study of solvent effects and properties of molecules in their liquid state or in solution. It is written in the FORTRAN90 programming language, and should be easy to follow, understand, maintain and modify. Given the nature of the ASEP/MD method, external programs are needed for the quantum calculations and molecular dynamics simulations. The present version of ASEP/MD includes interface routines for the GAUSSIAN package, HONDO, and MOLDY, but adding support for other programs is straightforward. This article describes the program and its usage.

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...

Solvent effects on the potential energy surface of the 1:1 complex of water and formamide: Application of the polarizable continuum model to the study of nonadditive effects

A study of the solvent effect on the potential energy surface of the 1:1 complex of water and formamide have been performed. In the description of the solvent we have employed the polarizable continuum model. The calculations were done at Hartree–Fock ab initio and Mo/ller–Plesset (MP) levels. We found that the geometry of the system is appreciably modified by the solvent. The most important changes are the inversion of the water molecule orientation and the increase of the O(formamide)–H(water) distance by about 0.2 A. In the gas phase binding to the carbonyl is energetically equivalent to binding to the amino group. However, in solution, water binds better to the carbonyl oxygen that to the NH group. The nonadditive contributions are, in general, important and can be related to the change in the monomer energies when one passes from the monomeric to the dimeric reaction potential.

Solvent effects on the transition of formaldehyde in liquid water. A QM/MM study using the mean field approximation

Abstract In this Letter we propose a new method for the study of solvent effects on electron spectra that combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. Unlike other QM/MM methods, the solvent perturbation is introduced into the solute molecular Hamiltonian in an averaged way, i.e., we use a mean field approximation. The method is implemented at the multiconfigurational self-consistent field and configuration interaction levels. Numerical results for the solvent shift of formaldehyde in liquid water are presented.

Multiconfigurational self-consistent and molecular mechanics simulation of solvent effects on the n→π∗ blue shift of pyrimidine ☆

Abstract The 1(n→π∗) electron transition of pyrimidine in liquid water was studied theoretically and the structure of the pyrimidine–water system was determined. The method combines multiconfigurational self-consistent quantum calculations in the description of the solute molecule with molecular dynamics calculations in the description of the solvent. It was shown that the solvent becomes more structured around the solute as the solute polarizes. The model adequately reproduces the experimental induced dipole moment and solvent shift. The contributions of the different components of the interaction energy to the solvent shift are also discussed.

A theoretical study of hydrogen-bonded complexes in solution: BSSE and decomposition of interaction energy

Abstract We present a procedure for computing the interaction energy and its variational components in hydrogen-bonded systems and for hydrated ions in solution. We examine averaged many-body effects due to the solvent, where this is modelled as a polarizable continuum. The theory is very general and is applicable to several solvent models. It is shown that, in general, the dimeric interaction potential changes appreciably from vacuum to solution. The origin of the non-additive contributions is related to the change in the monomeric energies in passing from the monomeric to the dimeric reaction potential.

Substituent and Solvent Effects on the UV–vis Absorption Spectrum of the Photoactive Yellow Protein Chromophore

Solvent effects on the UV-vis absorption spectra and molecular properties of four models of the photoactive yellow protein (PYP) chromophore have been studied with ASEP/MD, a sequential quantum mechanics/molecular mechanics method. The anionic trans-p-coumaric acid (pCA(-)), thioacid (pCTA(-)), methyl ester (pCMe(-)), and methyl thioester (pCTMe(-)) derivatives have been studied in gas phase and in water solution. We analyze the modifications introduced by the substitution of sulfur by oxygen atoms and hydrogen by methyl in the coumaryl tail. We have found some differences in the absorption spectra of oxy and thio derivatives that could shed light on the different photoisomerization paths followed by these compounds. In solution, the spectrum substantially changes with respect to that obtained in the gas phase. The n → π1* state is destabilized by a polar solvent like water, and it becomes the third excited state in solution displaying an important blue shift. Now, the π → π1* and π → π2* states mix, and we find contributions from both transitions in S1 and S2. The presence of the sulfur atom modulates the solvent effect and the first two excited states become practically degenerate for pCA(-) and pCMe(-) but moderately well-separated for pCTA(-) and pCTMe(-).