Gary S. Kedziora

Q-Chem 2.0: A High-Performance Ab Initio Electronic Structure Program Package

Q‐Chem 2.0 is a new release of an electronic structure program package, capable of performing first principles calculations on the ground and excited states of molecules using both density functional theory and wave function‐based methods. A review of the technical features contained within Q‐Chem 2.0 is presented. This article contains brief descriptive discussions of the key physical features of all new algorithms and theoretical models, together with sample calculations that illustrate their performance.

A SYSTEMATIC AB INITIO INVESTIGATION OF THE OPEN AND RING STRUCTURES OF OZONE

Abstract The energy difference between the open and the ring isomer of ozone as well as the dissociation energy ( O 3 ( X ,  1 A 1 ) → O 2 ( X ,  3 Σ − g )+ O ( 3 P )) have been determined at the CCSD(T), MR-CISD and MR-AQCC levels of theory. Using correlation consistent basis sets up to quintuple-zeta quality, the estimated complete basis set limits for CCSD(T) and MR-AQCC lie within 1 kcal/mol of the experimental value of 26.1±0.4 kcal/mol and place the ring isomer by 4.8 and 5.3 kcal/mol, respectively, above the dissociation limit. Zero-point vibrational corrections increase the latter two values by 1.4 kcal/mol.

The relativistic Dirac–Coulomb–Fock effect on atomization energies

We have used a second-order perturbation treatment of the Dirac–Coulomb–Hartree–Fock method to estimate relativistic contributions to energies in the G2/97 test set. The one-electron relativistic effect on atomization energies of molecules containing first- and second-row atoms nearly always reduces binding. When the relativistic corrections are included in G3 theory and assessed on the G2/97 test set, there is little change in overall performance.

CALCULATING DIPOLE AND QUADRUPOLE POLARIZABILITIES RELEVANT TO SURFACE ENHANCED RAMAN SPECTROSCOPY

Abstract We have used ab initio electronic structure calculations to calculate the frequency dependent dipole–dipole polarizability ( α ), quadrupole–dipole polarizability ( A ), quadrupole–quadrupole polarizability ( C ), (electric) dipole–magnetic dipole polarizability ( G ), and their normal coordinate derivatives for CO. The polarizability derivatives are of relevance to the interpretation of surface enhanced Raman (SER) spectra. Basis set convergence of these spectroscopic properties is studied, along with the effect of including electron correlation at the second-order polarization propagator (SOPPA) level, and the variation of the results with excitation frequency. The largest basis set SOPPA results that we have generated appear to be converged to within 20% or better for most of the properties we have studied, however in a few cases the convergence is much poorer. The most difficult property to converge involves the off-diagonal component of the derivatives of the quadrupole–dipole polarizability tensor. Our results show that the ratio of the largest components of A to the largest components of α are on the order of one atomic unit in size, and a similar statement can be made concerning the corresponding ratios of the normal coordinate derivatives. This means that the ratio of field derivatives to field strengths will also have to be on the order of one atomic unit in order for A and C to contribute comparably to α in determining SERS intensities.

M + Ng potential energy curves including spin-orbit coupling for M = K, Rb, Cs and Ng = He, Ne, Ar

The X(2)Σ(1/2)(+), A(2)Π(1∕2), A(2)Π(3∕2), and B(2)Σ(1/2)(+) potential energy curves and associated dipole matrix elements are computed for M + Ng at the spin-orbit multi-reference configuration interaction level, where M = K, Rb, Cs and Ng = He, Ne, Ar. Dissociation energies and equilibrium positions for all minima are identified and corresponding vibrational energy levels are computed. Difference potentials are used together with the quasistatic approximation to estimate the position of satellite peaks of collisionally broadened D2 lines. The comparison of potential energy curves for different alkali atom and noble gas atom combinations is facilitated by using the same level of theory for all nine M + Ng pairs.

Scalar relativistic effects on energies of molecules containing atoms from hydrogen through argon

Stationary direct perturbation theory is used to calculate a scalar relativistic correction to the species in the G3/99 test set. We observe that the relativistic energy almost always reduces atomization energies, electron affinities, and ionization potentials. Exceptions occur when s orbitals play a predominant role in the energy differences between reactants and products. The scalar relativistic energy, when added to G3 theory and the empirical parameters are reoptimized, gives slightly worse agreement with experiment.

Potential energy surfaces for alkali plus noble gas pairs: a systematic comparison

Optically Pumped Alkali Lasers (OPAL) involve interactions of alkali atoms with a buffer gas typically consisting of a noble gas together with C2H4. Line broadening mechanisms are of particular interest because they can be used to match a broad optical pumping source with relatively narrow alkali absorption spectra. To better understand the line broadening processes at work in OPAL systems we focus on the noble gas collisional partners. A matrix of potential energy surfaces (PES) has been generated at the multi-configurational self consistent field (MCSCF) level for M + Ng, where M=Li, Na, K, Rb, Cs and Ng=He, Ne, Ar. The PES include the X2Σ ground state surface and the A2II, B2Σ excited state surfaces. In addition to the MCSCF surfaces, PES for Li+He have been calculated at the multi-reference singles and doubles configuration interaction (MRSDCI) level with spin-orbit splitting effects included. These surfaces provide a way to check the qualitative applicability of the MCSCF calculations. They also exhibit the avoided crossing between the B2Σ and A2II1/2 surfaces that is partially responsible for collision induced relaxation from the 2P3/2 to the 2P1/2 atomic levels.