The Criegee intermediates are carbonyl oxides postulated to play key roles in the reactions of ozone with unsaturated hydrocarbons; these reactions constitute an important mechanism for the removal of unsaturated hydrocarbons and for the production of OH in the atmosphere. Here, we report the transient infrared (IR) absorption spectrum of the simplest… (More)
The Zhang–Zhang(ZZ) polynomials (aka Clar covering polynomial) for several subclasses of catacondensed and pericondensed benzenoid systems have been computed using an automatic computer code developed in our group and described in [C.-P. Chou and H.A. expressions for several series of catacondensed benzenoids and for the prolate rectangular pericondensed… (More)
Closed-form, general formulas for the Zhang-Zhang (ZZ) polynomials for two important classes of benzenoid structures, chevrons ݄ܥሺ݇ǡ ݉ǡ ݊ሻ and generalized chevrons ݄ܥሺ݇ǡ ݉ǡ ݊ ଵ ǡ ݊ ଶ ሻ, are formally derived. The derivations rely on a new and important theorem, which states that the ZZ polynomial of two fused parallelograms can be represented as the… (More)
We employ a graphical proof-oriented tool, ZZDecomposer, to discover formal derivations of Zhang-Zhang (ZZ) polynomials for various families and subfamilies of benzenoid structures including tripods, zigzag-edge coronoids fused with a starphene, oblate rectangles ܱݎሺ݉ǡ ʹሻ, decompositions of the analyzed structures. The decompositions provide appropriate… (More)
Prediction of a false ground state with popular variants of multireference perturbation theory (CASPT2 and MRMP) is reported for a remarkably simple chemical system: the Sc(2) molecule.
A detailed analysis of a severe intruder state problem in the multistate multireference perturbation theory (MS-MRPT) calculations on the ground state of manganese dimer is presented. An enormous number of detected intruder states (> 5000) do not permit finding even an approximate shape of the X(1)Sigma(g) (+) potential energy curve. The intruder states are… (More)
The second-order multireference perturbation theory using an optimized partitioning, denoted as MROPT(2), is applied to calculations of various molecular properties-excitation energies, spectroscopic parameters, and potential energy curves-for five molecules: ethylene, butadiene, benzene, N(2), and O(2). The calculated results are compared with those… (More)
A new perturbation approach is proposed that enhances the low-order, perturbative convergence by modifying the zeroth-order Hamiltonian in a manner that enlarges any small-energy denominators that may otherwise appear in the perturbative expansion. This intruder state avoidance (ISA) method can be used in conjunction with any perturbative approach, but is… (More)