Daniel M. Chipman

Reaction field treatment of charge penetration

Treatment of the important electrostatic effects of solvation by means of reaction field theory is becoming common in electronic structure calculations on molecules. Most extant reaction field methods neglect or crudely approximate the often important influence of volume polarization arising from solute charge that quantum mechanically penetrates outside the cavity that nominally encloses it. This work proposes and examines a new formulation that provides an accurate simulation of volume polarization effects while being much simpler to implement and use than an exact treatment. Detailed comparisons with other related methods are also given.

Gaussian basis sets for calculation of spin densities in first-row atoms

SummaryThe suitability of Gaussian basis sets for ab initio calculation of Fermi contact spin densities is established by application to the prototype first-row atoms B-F having open shell p electrons. Small multiconfiguration self-consistent-field wave functions are used to describe relevant spin and orbital polarization effects. Basis sets are evaluated by comparing the results to highly precise numerical grid calculations previously carried out with the same wave function models. It is found that modest contracted Gaussian basis sets developed primarily for Hartree-Fock calculations can give semiquantitative results if augmented by diffuse functions and if further uncontracted in the outer core-inner valence region.

Volume polarization in reaction field theory

In continuum reaction field models of solvation, unconstrained quantum mechanical calculation of the solute electronic structure inevitably leads to penetration of some solute charge density outside the cavity and into the solvent dielectric region. This produces a rarely recognized or treated volume polarization that contributes in addition to the commonly considered surface polarization. In this work a new practical implementation is described for quantitatively evaluating both volume and surface polarization contributions to the solute-solvent interaction with an irregularly shaped cavity surface. For illustration, numerical results are presented on several representative small neutral, cation, and anion solutes. The volume polarization contributions to energies and dipole moments are found to be somewhat smaller than those from surface polarization, but not negligible. The results are also used to test several charge renormalization approaches that have been previously proposed in the literature. Comp...

CHARGE PENETRATION IN DIELECTRIC MODELS OF SOLVATION

Dielectric continuum models are widely used for treating solvent effects in quantum chemical calculations of solute electronic structure. These invoke a reaction field wherein solute-solvent electrostatic interactions are explicitly or implicitly described by means of certain apparent polarization charges. Most implementations represent this polarization through an apparent surface charge distribution spread on the boundary of the cavity that nominally encloses the solute. However, quantum chemical calculations usually lead to a tail of the wave function penetrating outside the cavity, thereby causing an additional volume polarization contribution to the reaction field that is rarely recognized or treated. In principle the volume polarization should be represented by a certain apparent volume charge distribution spread throughout the entire dielectric medium. It is shown here that this effect can be closely simulated by means of a certain additional apparent surface charge distribution. This provides a co...

Calculation of spin densities in diatomic first-row hydrides

Fermi contact spin densities have been theoretically determined for the ground‐state diatomic first‐row hydrides CH, OH, and NH having open shell π electrons. Multiconfiguration self‐consistent‐field wave functions include the dominant configuration and single excitations from it describing the most important spin and orbital polarization effects. Optimization of the orbitals by precise numerical grid methods shows that this simple wave function model is capable of providing spin densities in satisfactory agreement with experiment. Gaussian basis sets suitable for use with this wave function model are determined by comparing to the precise numerical spin density results. Huzinaga’s popular (9s5p‖4s) primitive Gaussian basis provides a good starting point if augmented with diffuse and polarization functions and with a tight (high exponent) s function at hydrogen. Only the innermost few primitive functions may be contracted. Contraction coefficients may be determined on the basis of free atom Hartree–Fock c...

The spin polarization model for hyperfine coupling constants

SummaryThe simple spin polarization model for calculation of the spin densities that determine hyperfine coupling constants in free radicals is examined. Spin-unrestricted approaches, both without and with removal of spin contamination, are discussed and compared with spin-restricted treatments. Basis set design and electron correlation effects are also considered. Calculations on small pi radicals are presented to illustrate the arguments. Explanations are advanced for why some of the simpler treatments seem to work better than might be expected.

Theoretical study of the properties of methyl radical

A detailed ab initio study of the structure and properties of methyl radical in its ground electronic state is presented. The primary focus is on the spin density distribution and its dependence on basis set, wave function model, and molecular geometry. By using a natural orbital analysis that provides an unambiguous decomposition into core and valence contributions, interesting theoretical and computational results are found that clarify the relationships among CI, UHF, and PUHF models. In particular, it appears that the better agreement with experiment for the PUHF model as compared to UHF is simply fortuitous. A large and negative core contribution found in the 13C isotropic hyperfine coupling constant explains why minimal basis set calculations always greatly overestimate this property, regardless of what wave function model is used. All the models studied give good results for anisotropic hfc and for the vibrational corrections to isotropic hfc, although not for the isotropic hfc themselves. Through ...

Cavity size in reaction field theory

The optimum size of the cavity accommodating a solute in the reaction field theory of solvation is considered by empirical calibration of the results of electronic structure calculations against experiment. To isolate the long range electrostatic free energy contributions treated by reaction field theory from the many other short range contributions not explicitly considered, computational results are compared to experimental determinations of conformational free energy differences in polar solutes having two or more stable or metastable isomers. When the cavity shape is defined by a solute electronic isodensity contour, it is found that the best overall agreement with experiment is obtained with a cavity size corresponding to the 0.001 a.u. contour.

Excited electronic states of small water clusters

The lowest electronic states that are initially formed upon excitation of small water clusters having a central water molecule with one stretched OH bond are studied with electronic structure methods. It is found that in water dimer, trimer, and pentamer the lowest excited singlet and triplet states are each nondissociative for stretching of an OH bond that is hydrogen bonded in an icelike configuration to a neighboring water molecule. This is in marked contrast to the behavior of an isolated gas phase water monomer, where it is well known that the lowest excited state is strongly dissociative upon OH stretching. The conclusions of this study may serve as a basis to interpret recent experimental evidence that suggests a significant lifetime for excited water in irradiated thin ice films, and may also have important implications for the behavior of excitation of liquid water.

Simulation of volume polarization in reaction field theory

In the reaction field theory of solvation, penetration of charge density outside the cavity nominally enclosing the solute leads to a volume polarization that contributes in addition to the commonly recognized surface polarization. In principle the exact volume polarization charge density is spread everywhere outside the cavity, but its effect can be closely and concisely simulated by a certain additional surface polarization charge density. Formal comparison is made to conductorlike screening models, and it is found that these improve on common approaches that neglect volume polarization by automatically including the simulation of volume polarization. A revised method to numerically determine this simulation is also described.