Anders Öhrn

A theoretical study of the solvent shift to the n -> pi* transition in formaldehyde with an effective discrete quantum chemical solvent model including non-electrostatic perturbation

An effective solvent model with an explicit solvent representation is described. The modelled perturbation of the solute due to the discrete solvent molecules includes polarization and a non-electrostatic interaction. The latter depends on the overlap between the solute wave function and the solvent orbitals and approximately accounts for the restraint the Pauli principle puts on the space which the solute wave function is allowed to occupy, and consequently also models the exchange repulsion between solute and solvent. The wave function of the solute is a linear combination with variational coefficients of orthogonal states obtained with the complete active space state interaction (CASSI) method. With this model, the solvent shift to the n→ π* transition in hydrated formaldehyde is studied and the analysis of the results investigates the different contributions to the total shift; the non-electrostatic interaction is found to be of significance and capable of coupling with the electrostatic interaction in qualitatively different ways for the ground and first excited state. Solvent density distributions for hydrated formaldehyde are also reported.

Accuracy of typical approximations in classical models of intermolecular polarization

One of the largest limitations of standard molecular-mechanics force fields is the neglect of intermolecular polarization. Several attempts to cure this problem have been made, but the results have not always been fully satisfactory. In this paper, we present a quantitative study of the fundamental approximations that underlie polarization models for classical force fields. The induced charge density of a large set of molecular dimers is compared to supermolecular calculations for a hierarchy of simplified models. We study the effect of the Pauli principle, the local inhomogeneity of the electric field, the intramolecular coupling of the polarization response, and the fact that the induced density is a continuous function. We show that standard point-polarizability models work rather well, despite their lack of all these effects, because (1) there is a systematic error cancellation between the neglect of effects of the Pauli principle and the locally inhomogeneous electric field, and (2) the lack of intramolecular coupling and the use of a dipole expansion of the induced density have only minor effects on the polarization. However, the cancellation in (1) is not perfect, and therefore polarizable force-fields could be improved if both effects are explicitly treated.

Many-Body Polarization, a Cause of Asymmetric Solvation of Ions and Quadrupoles.

Three models are used to study the effect of many-body polarization in the solvation of non-dipolar molecules and ions in water. Two of the models are very simplified and are used to show a number of basic principles of correlation of solvent degrees of freedom and asymmetric solvent structures. These principles are used to interpret results from the third model:  an accurate simulation of para-benzoquinone (PBQ) in aqueous solution with a combined quantum chemical statistical mechanical solvent model with an explicit solvent. It is found that nonzero polarizability of PBQ introduces correlation in the solvent degrees of freedom through the many-body nature of the polarization. The fluctuating electric field from the solvent on the solute increases in magnitude with the correlation. Solvent effects are hence modified. This correlation is not described within the mean-field approximation. In practice, the correlation will show up as an increased probability for asymmetric solvation of the molecule.

Method for Slater-Type Density Fitting for Intermolecular Electrostatic Interactions with Charge Overlap. I. The Model

The effects of charge overlap, or charge penetration, are neglected in most force fields and interaction terms in QM/MM methods. The effects are however significant at intermolecular distances near the van der Waals minimum. In the present study, we propose a method to evaluate the intermolecular Coloumb interaction using Slater-type functions, thus explicitly modeling the charge overlap. The computational cost of the method is low, which allows it to be used in large systems with most force fields as well as in QM/MM schemes. The charge distribution is modeled as a distributed multipole expansion up to quadrupole and Slater-type functions of angular momentum up to L = 1. The exponents of the Slater-type functions are obtained using a divide-and-conquer method to avoid the curse of dimensionality that otherwise is present for large nonlinear optimizations. A Levenberg-Marquardt algorithm is applied in the fitting process. A set of parameters is obtained for each molecule, and the process is fully automated. Calculations have been performed in the carbon monoxide and the water dimers to illustrate the model. Results show a very good accuracy of the model with relative errors in the electrostatic potential lower than 3% over all reasonable separations. At very short distances where the charge overlaps is the most significant, errors are lower than 8% and lower than 3.5% at distances near the van der Waals minimum.

An explicit quantum chemical solvent model for strongly coupled solute–solvent systems in ground or excited state

A detailed account of the explicit quantum chemical solvent model QMSTAT is given. The model is presented in terms of three coupled aspects of relevance for all types of quantum chemical solvent models: the quantum chemical method, the intermolecular interactions and the statistical mechanical method. The quantum chemical method is either a compact natural orbital formulation of the standard Hartree–Fock method or a compact multiconfigurational method with a state basis. The latter method can describe excited states apart from the ground state and is for most systems an excellent approximation to the complete active space self-consistent field method. Both static and induced electrostatic interaction terms between the quantum chemical region and the solvent are included. Further, a non-electrostatic term is added to describe effects which derive from the Pauli principle. This term models both the exchange repulsion between solute and solvent and the packing effects an environment has on a molecule, in particular on diffuse states of the molecule. The statistical mechanical problem is solved with an exact Metropolis–Monte Carlo simulation that requires several similar quantum chemical problems to be solved. Since the quantum chemical problem and the statistical mechanical problem are solved as a coupled problem, the present model is especially useful for problems where electronic degrees of freedom of the solute strongly depend on the solvent distribution and vice versa. Three applications are summarized, which highlight this type of coupling present in QMSTAT and the non-electrostatic contribution. The examples are the solvation of four monatomic ions, the solvation of para-benzoquinone and the solvation of indole and the solvent shift to its absorption and fluorescence spectra