N. W. Winter

Ab initio effective core potentials including relativistic effects. II. Potential energy curves for Xe2, Xe+2, and Xe*2

Potential energy curves for the ground 1Σ+g state of Xe2, the first four states of the Xe+2 ions, and the eight Xe*2 excimer states corresponding to the addition of a 6sσg Rydberg electron to these ion cores have been computed using averaged relativistic effective core potentials (AREP) and the self‐consistent field approximation for the valence electrons. The calculations were carried out using the LS‐coupling scheme with the effects of spin–orbit coupling included in the resulting potential energy curves using an empirical procedure. A comparison of nonrelativistic and averaged relativistic EP’s and subsequent molecular calculations indicates that relativistic effects arising from the mass–velocity and Darwin terms are not important for these properties of Xe2 molecules. Spectroscopic constants for Xe+2 are in good agreement with all electron CI calculations suggesting that the computed values for Xe*2 excimers should be reliable. The lifetime for the Ou+ state of the Xe2* is computed to be 5.6 nsec whi...

Theoretical study of a Cu+ ion impurity in a NaF host

The Cu+ ion impurity in a NaF host has been modeled using a finite cluster of ions to represent the crystal lattice. Several approximations to the lattice potential in the region of the cluster were compared to the exact Madelung potential. The error in the calculated nearest‐neighbor distance for the pure host was found to be proportional to the error in the lattice potential. Hartree–Fock calculations were carried out for the ground 1A1g and excited 1,3Eg and 1,3T2g states of the NaF:Cu+ system. The resulting energy level structure was compared to the experimental spectra. The symmetric‐stretch potential energy curve, vibrational frequencies, and Franck–Condon factors were calculated for the 1A1g and 1,3T2g states. Using a single configuration coordinate model and a semiempirical spin–orbit coupling scheme, the relative intensities and bandwidths were calculated for absorption to the 1,3T2g states and compared to experiment.

Defect evolution and pore collapse in crystalline energetic materials

This work examines the use of crystal based continuum mechanics in the context of dynamic loading. In particular, we examine model forms and simulations which are relevant to pore collapse in crystalline energetic materials. Strain localization and the associated generation of heat are important for the initiation of chemical reactions in this context. The crystal mechanics based model serves as a convenient testbed for the interactions among wave motion, slip kinetics, defect generation kinetics and physical length scale. After calibration to available molecular dynamics and single crystal gas gun data for HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), the model is used to predict behaviors for the collapse of pores under various conditions. Implications for experimental observations are discussed.

Improved ab initio effective potentials for Ar, Kr, and Xe with applications to their homonuclear dimers

Effective core potentials for the Ar, Kr, and Xe atoms derived from numerical Hartree–Fock and Dirac–Hartree–Fock wave functions are applied in SCF and CI calculations of homonuclear diatomic potential energy curves. Detailed comparisons are made with the all‐electron calculations of Wadt for the ground and lowest positive states. Relativistic effects, excluding spin–orbit coupling, are seen to be relatively unimportant. Plots of the potential energy curves and computed spectroscopic constants show excellent agreement with the all‐electron results. On the other hand, comparisons with results obtained using effective potentials derived using varients of Phillips–Kleinman procedures show dramatic differences for Xe2 and Xe2+. From SCF calculations on Xe2 and Xe2+ it was found that the explicit inclusion of the spin–orbit operator in the SCF procedure (using ω–ω coupling) results in essentially the same potential curves obtained by adding the spin–orbit correction as a final semiempirical perturbation.

Configuration interaction calculation of the electronic spectra of MgF2:V+2

The ground and excited quartet states of divalent vanadium in a magnesium fluoride host have been determined from Hartree–Fock and configuration‐interaction calculations. The potential energy curves for the symmetric stretch of the nearest‐neighbor fluoride ions were determined and used to calculate the vibrational energy levels and wave functions. The ground‐state absorption bandwidths were calculated from the Franck–Condon overlaps and compared to the experimental low‐temperature spectra. The calculated vertical excited‐state absorption energies from the first excited state are also found to be in good agreement with recent measurements.

Ab initio and Gordon–Kim intermolecular potentials for two nitrogen molecules

Both ab initio MO–LCAO–SCF and the electron‐gas (or Gordon–Kim) methods have been used to compute the intermolecular potential (Φ) of N2 molecules for seven different N2–N2 orientations. The ab initio calculations were carried out using a [4s3p] contracted Gaussian basis set with and without 3d polarization functions. The larger basis set provides adequate results for Φ≳0.002 hartree or intermolecular separations less than 6.5–7 bohr. We use a convenient analytic expression to represent the ab initio data in terms of the intermolecular distance and three angles defining the orientations of the two N2 molecules. The Gordon–Kim method with Rae’s self‐exchange correction yields Φ, which agrees reasonably well over a large repulsive range. However, a detailed comparison of the electron kinetic energy contributions shows a large difference between the ab initio and the Gordon–Kim calculations. Using the ab initio data we derive an atom–atom potential of the two N2 molecules. Although this expression does not a...

Accurate characterization of the transition state geometry for the HF+H'. -->. H+H'F reaction

The potential surface for the exchange reaction HF+H′→H+H′F has been investigated with various multiconfiguration self‐consistent field (SCF) and configuration interaction (CI) wavefunctions. For the first time, nonlinear geometries have been considered. The calculations demonstrate the importance of diffuse functions on the fluorine for describing nonlinear geometries. A qualitative model is presented to explain the need for diffuse functions. The energy barrier for the exchange reaction is calculated to be ∼45 kcal/mole, which is comparable to the values obtained in previous ab initio calculations on the collinear reaction surface (Refs.1 and 2). More importantly, the calculations show that the saddle point region is very flat, the barrier changing by only 1–2 kcal/mole between collinear (180°) and perpendicular (90°) geometries. The optimum angle for the transition state geometry is calculated to be 106°. Dynamics calculations on the HF+H′→H+H′F reaction have employed semiempirical London–Eyring –Polan...

Hartree--Fock calculation of the electronic structure of a Cu impurity in NaCl

Hartree–Fock cluster calculations have been carried out for the ground 3d10 and excited 3d94s configurations of the Cu+ ion in a NaCl host. Special emphasis has been given to providing an accurate representation of the Coulomb potential due to the remainder of the lattice. Configuration coordinate curves were determined for the symmetric displacement of the nearest‐neighbor Cl− ions and are compared to recent Xα calculations. The Hartree–Fock equilibrium Cu–Cl distance was found to be 5.327 bohr, slightly shorter than the calculated nearest‐neighbor distance of 5.353 bohr for the pure NaCl host. Comparison of the Hartree–Fock and Xα ground and excited state energies, shows that the Xα approximation reverses the ordering of the 3T2g and 1Eg states, overestimates the equilibrium nearest‐neighbor distance, and predicts the a1g vibrational frequency to be about twice the Hartree–Fock value. Using the Franck–Condon factors found with the ab initio potential energy curves, the calculated bandwidths for the 1,3T...

Kinetics and thermodynamic behavior of carbon clusters under high pressure and high temperature

Physical processes that govern the growth kinetics of carbon clusters at high pressure and high temperature are: (a) thermodynamics and structural sp ?-to- sp ? bonding) changes and (b) cluster diffusion. Our study on item (a) deals with ab initio and semi-empirical quantum mechanical calculations to examine effects of cluster size on the relative stability of graphite and diamond clusters and the energy barrier between the two. We have also made molecular dynamics simulations using the Brenner bond order potential. Kesults show that the melting line of diamond based on the Brenner potential is reasonable and that the liquid structure changes from mostly sp -bonded carbon chains to mostly sp ?-bonding over a relatively narrow density interval. Our study on item (b) uses the time-dependent clustor size distribution function obtained from the relevant Smoluchowski equations. The resulting surface contribution to the Gibbs free energy of carbon clusters was implemented in a thermochemical code.

Ab initio potential energy curves for the low-lying electronic states of the argon excimer

Configuration interaction calculations are reported for the potential energy curves of the argon excimer that arise due to excitation to the 4s and 4p Rydberg molecular orbitals. Effective core potentials were employed to replace the core electrons of the Ar atoms thereby reducing the computational procedure to one for a 16 valence system. Potential energy curves for three excimer states of each of the symmetries 1Σ+g, 1Σ+u, 3Σ+g, 3Σ+u, 1Πu, 3Πu, 1Πg, and 3Πg are reported and compared with those previously computed. Spectroscopic constants and curve maxima are reported where appropriate.