Eugene V. Stefanovich

A new method for incorporating solvent effect into the classical, ab initio molecular orbital and density functional theory frameworks for arbitrary shape cavity

Abstract A new method for calculating the solvation energy of an arbitrary shape solute is presented. In this method, the solvents is treated as a homogeneous dielectric medium with a cavity. The solvation energy is presented in the Hartee-Fock-Roothaan form, which can be incorporated into both molecular orbital and density functional theories, as well as in the classical theory using the distributed monopole approach. We found that this approach yields on the average of 10% additional CPU time compared to the gas-phase calculations and an accuracy of better than 2.0 kcal/mol for neutral polar solutes but somewhat larger for ions.

An excitonic mechanism of detonation initiation in explosives

A novel mechanism for detonation initiation in solid explosives is proposed. This is based on electronic excitations induced by an impact wave propagating through the crystal. We illustrate the model by using the RDX (C3H6N6O6) crystal as an example. In our model, a key role belongs to lattice defects, in particular edge dislocations, which promote dramatic changes in the electronic structure, primarily a reduction of the optical gap due to the splitting off of local electronic states from both valence and conduction bands. The pressure inside the impact wavefront further reduces the band gap, making it close to zero. This promotes highest occupied molecular orbital–lowest unoccupied molecular orbital HOMO–LUMO transitions resulting in N–NO2 bond breaking and the creation of favorable conditions for the initiation of a chain reaction. Experimental facts supporting the suggested mechanism are discussed.

OPTIMIZED ATOMIC RADII FOR QUANTUM DIELECTRIC CONTINUUM SOLVATION MODELS

Abstract We present an optimized set of atomic radii for H, C, N, O, F, P, S and Cl, which can be used in quantum dielectric continuum calculations. These atomic radii yield differences in hydration energies with experimental data of about 1 kcal/mol for neutral molecules, 2 kcal/mol for cations and 2–5 kcal/mol for anions at the Hartree-Fock, second order Moller-Plesset perturbation, and various non-local density functional theories within the framework of the generalized conductor-like screening model. Almost the same accuracy was also found when these radii were used with the polarizable continuum model (PCM). The accuracy of the recently proposed self-consistent isodensity PCM model is also discussed.

Ab initio study of water adsorption on TiO2(110): molecular adsorption versus dissociative chemisorption

Abstract Ab initio embedded cluster calculations of water interaction with the TiO 2 (110) surface predict that molecular adsorption is more favorable than dissociative chemisorption. Unlike all previous calculations, this result is in agreement with recent temperature-programmed desorption and modulated beams experiments. Calculated surface relaxation, water adsorption energy and the densities of electronic states are also in good agreement with available experimental data. This agreement suggests that the embedded cluster model developed in this work is an accurate predictive tool that can be useful for theoretical studies of photocatalytic reactions on titanium dioxide surfaces.

Embedded density functional approach for calculations of adsorption on ionic crystals

We present an embedded density functional approach to study adsorption on crystalline surfaces. Following ideas suggested by Cortona, Wesolowski, and Warshel, we divide the total system into a quantum cluster and the surrounding lattice whose density is assumed to be the same as in the ideal crystal. In this case the Kohn–Sham Hamiltonian for electrons in the cluster contains additional terms corresponding to the Coulomb, exchange, correlation, and ‘‘nonadditive kinetic energy’’ potentials from the environment. Test calculations for the He and Ar dimers, X–H2O molecular complexes (X=Li+, Na+, K+, F− or Cl−) and water adsorption on the (001) surface of the NaCl crystal suggest that this model provides a promising alternative for cluster models employed earlier for calculations of defects and adsorption on ionic crystals.

Analytical first and second energy derivatives of the generalized conductorlike screening model for free energy of solvation

We present analytical expressions for the first and second energy derivatives of our recently proposed generalized conductorlike screening model (GCOSMO) for free energy of solvation of solute in an arbitrary shape cavity. An application to study hydration effects on structure and stability of glycine zwitterion in aqueous solution is also presented. These calculations were carried out at the Hartree–Fock, second‐order Mo/ller–Plesset perturbation theory and different nonlocal density functional theory levels using the 6‐31G(d,p) basis set. We found that our quantum mechanical GCOSMO solvation model costs from 10% to 40% extra cpu time per one Berny optimization step compared to the gas‐phase calculations for different levels of theory. For the glycine system, the optimized zwitterionic structure in aqueous solution agrees very well with experimental crystal structure and the enthalpy change for transfering glycine from the gas phase to the aqueous solution is also in excellent agreement with experimental...

A general methodology for quantum modeling of free-energy profile of reactions in solution: An application to the Menshutkin NH3+CH3Cl reaction in water

We present a general methodology for calculating free-energy profile of reaction in solution using quantum mechanical methods coupled with the dielectric continuum solvation approach. Particularly, the generalized conductorlike screening model (GCOSMO) was employed in this study, though any continuum model with existing free-energy derivatives could also be used. Free-energy profile is defined as the steepest descent path from the transition state to the reactant and product channels on the liquid-phase free-energy surface. Application of this methodology to calculate the free-energy profile of the Menshutkin NH3+CH3Cl reaction in water is discussed. The efficiency of the GCOSMO method allows characterization of stationary points and determination of reaction paths to be carried out at less than 20% additional computational cost compared to gas-phase calculations. Excellent agreement between the present results and previous Monte Carlo simulations using a combined quantum mechanical/molecular mechanics (QM/MM) potential confirms the accuracy and usefulness of the GCOSMO model.We present a general methodology for calculating free-energy profile of reaction in solution using quantum mechanical methods coupled with the dielectric continuum solvation approach. Particularly, the generalized conductorlike screening model (GCOSMO) was employed in this study, though any continuum model with existing free-energy derivatives could also be used. Free-energy profile is defined as the steepest descent path from the transition state to the reactant and product channels on the liquid-phase free-energy surface. Application of this methodology to calculate the free-energy profile of the Menshutkin NH3+CH3Cl reaction in water is discussed. The efficiency of the GCOSMO method allows characterization of stationary points and determination of reaction paths to be carried out at less than 20% additional computational cost compared to gas-phase calculations. Excellent agreement between the present results and previous Monte Carlo simulations using a combined quantum mechanical/molecular mechanics (Q...

Nature of the excited states of the rutile TiO2(110) surface with adsorbed water

We present an ab initio embedded cluster study of the ground and excited states for the rutile (1 1 0) surface with adsorbed water molecule. The calculated adsorption energies and geometries are compared with available experimental data. Upon photoexcitation,an electron–hole pair is created. We found that the electron is localized in the subsurface and thus cannot be directly involved in surface processes whereas the hole is localized on the surface oxygen atoms and enhances the radical character of the hydroxyl group. Our results support experimental suggestions that OH radical is the active species in photocatalysis on TiO2 surface. 2001 Elsevier Science B.V. All rights reserved.

An ab initio study of solvent shifts in vibrational spectra

Using analytical second derivatives of the generalized conductorlike screening model (GCOSMO) we calculate vibrational frequency shifts for several molecules (acetone, methylamine, formic acid, acetic acid, and trans‐NMA) solvated in water. In these calculations, results from dielectric continuum approach with and without several explicit water molecules are compared with traditional supermolecule approach. The simple GCOSMO model, where all solvent molecules are treated as a continuum medium, reproduces quite accurately solvent shifts in solutes having moderate hydrogen bondings with water, such as acetone and methylamine. To represent strong solvent effects in formic acid and acetic acid, one should add at least one explicit water molecule in GCOSMO calculations. Solvent effects on solute structure correlate well with frequency shifts. Geometry optimizations and frequency calculations in the GCOSMO‐supermolecule approach require only 10%–20% more computational effort than similar calculations in the gas...

Development of a perturbative approach for Monte Carlo simulations using a hybrid ab initio QM/MM method

Abstract We present the development and assessment on the accuracy of perturbative approach for Monte Carlo simulations using a hybrid ab initio quantum mechanical/molecular mechanics (QM/MM) potential. The central idea of this approach is that for most solvent moves, the changes in the solute wavefunction are small, thus one can use the perturbation theory to approximate the energy change without having to perform full quantum calculation at each Monte Carlo step. Consequently, the computational demand can be reduced by several orders of magnitude while maintaining a reasonable level of accuracy. This opens new possibilities for using more accurate levels of theory to describe both solute and solvent molecules in Monte Carlo simulations.