M.E. Martı́n

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.

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 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 solvent effects on the 1(n→π*) electron transition in acrolein

The (1)(n-->pi(*)) electron transition of acrolein in liquid water was studied theoretically by using the averaged solvent electrostatic potential/molecular dynamics method. The model combines a multireference perturbational treatment in the description of the solute molecule with molecular dynamics calculations in the description of the solvent. We demonstrate the importance of the solvent electron polarization, bulk solvent effects, and the use of relaxed geometries in solution on the calculated solvent shift. It is also shown that the inclusion of the dynamic correlation does not change the solvent shift although it must be used to reproduce the transition energy.

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.

An iterative procedure to determine Lennard-Jones parameters for their use in quantum mechanics/molecular mechanics liquid state simulations

Abstract A new procedure to optimize Lennard-Jones (LJ) parameters for liquid state simulations using combined ab initio quantum mechanical and molecular mechanical potentials (QM/MM) is described. In this paper, the LJ parameters are determined in an iterative way taking advantage of the similarity between QM/MM simulations and computations using the averaged solvent electrostatic potential (ASEP) QM/MM approach. The procedure is illustrated through the study of structural and thermodynamic properties of liquid water. The optimized parameters improve significantly the results of previous QM/MM simulations at the same theoretical level.

A comparative study of two QM/MM methods testing the validity of the mean field approximation

Abstract We compared the performances of two methods that combine quantum mechanics and molecular mechanics for the study of liquid systems. They differ in the description of the solute–solvent interaction. One makes use of the mean field approximation and the other does not. We show that the introduction of this approximation does not introduce significant errors into the induced dipole moment of the solute, the interaction energy or the solvent structure, while it permits a considerable reduction (from several thousand to four or five) of the number of quantum calculations and hence of the computational demands.

Comparison of three effective Hamiltonian models of increasing complexity: triazene in water as a test case.

A critical comparison of widely used solvation models is reported. It is illustrated by a study of the triazene molecule in liquid water. We consider the following approaches: (1) a continuum model based on multicentric multipole expansions of the charge distribution, (2) the averaged solvent electrostatic potential from molecular dynamics (ASEP/MD) method, and (3) molecular dynamics simulations using a combined quantum mechanics/molecular mechanics potential (QM/MM/MD). We find that the solvation induces appreciable changes in the geometry and charge distribution of triazene. These changes are only qualitatively reproduced by the dielectric continuum model, which clearly underestimates induced dipole moments and solute-solvent interaction energy. We also show that the use of effective point charges placed on solute nuclei during the classical simulations may cause significant errors in the description of the solvent structure. The addition of charges representing nitrogen atom lone pairs is compulsory to reproduce the QM/MM/MD simulation results. Moreover, our results validate the use of the mean field approximation in the study of solvent effects. A major conclusion of this study is that the ASEP/MD method constitutes a reliable alternative to the much more computationally demanding QM/MM/MD methods.

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(-).