State-of-the-art electronic structure calculations (MR-CISD) are used to map five different dissociation channels of CH3Cl along the C-Cl coordinate: (i) CH3(X̃(2)A2″) + Cl((2)P), (ii) CH3(3s(2)A1') + Cl((2)P), (iii) CH3(+)((1)A1') + Cl(-)((1)S), (iv) CH3(3p(2)E') + Cl((2)P), and (v) CH3(3p(2)A2″) + Cl((2)P). By the first time these latter four dissociation channels, accessible upon VUV absorption, are described. The corresponding dissociation limits, obtained at the MR-CISD+Q level, are 3.70, 9.50, 10.08, 10.76, and 11.01 eV. The first channel can be accessed through nσ* and n3s states, while the second channel can be accessed through n(e)3s, n(e)3p(σ), and σ3s states. The third channel, corresponding to the CH3(+) + Cl(-) ion-pair, is accessed through n(e)3p(e) states. The fourth is accessed through n(e)3p(e), n(e)3p(σ), and σ3p(σ), while the fifth through σ3p(e) and σ(CH)σ* states. The population of the diverse channels is controlled by two geometrical spots, where intersections between multiple states allow a cascade of nonadiabatic events. The ion-pair dissociation occurs through formation of CH3(+)···Cl(-)and H2CH(+)···Cl(-) intermediate complexes bound by 3.69 and 4.65 eV. The enhanced stability of the H2CH(+)···Cl(-) complex is due to a CH···Cl hydrogen bond. A time-resolved spectroscopic setup is proposed to detect those complexes.

The concerted and stepwise mechanisms of the Diels-Alder reaction between 1,3-butadiene and ethene have been investigated using highly correlated multireference methods (MRAQCC) and extended basis sets. Full MRAQCC geometry optimizations have been performed in all cases. The best estimate for the energy barrier of the Diels-Alder reaction is 22 kcalmol(-1). Anti- and gauche-out minima for the biradical structures and corresponding fragmentation saddle points have been determined. The biradical anti fragmentation saddle point is located 6.5 kcalmol(-1) above the concerted saddle point. The gauche-in structure does not correspond to a local minimum, but leads on geometry optimization directly to cyclohexene.

Valence-excited singlet (S1,S2) and triplet (T1–T4) states of acetylene have been studied by means of extended multireference electron correlation techniques (MR-CISD, MR-CISD+Q, and MR-AQCC). Extrapolations to the basis set limit have been performed. Minima and saddle points have been calculated using a recently developed analytic gradient method for excited states. Planar as well as nonplanar structures have been considered. In particular, the existence of an asymmetric, planar cis-type minimum on the S2 surface has been confirmed conclusively. Moreover, an intersection S1/S2 has been located close to this minimum. This situation will most probably affect the interpretation of the absorption bands attributed to the trans 1 1Bu state. In-plane and out-of-plane saddle points for cis–trans isomerization have been determined and characterized by harmonic vibrational analysis. Several interesting surface crossings for different electronic states (S1/S2, T2/T3, and S1/T3) have been characterized. Implications of the flatness of the T3 surface around linear structures and the location of the S1/T3 crossing seam on the anomalities observed in the ZAC spectrum of the Ã 1Au state are discussed.

We introduce an alternate model to describe twistons in crystalline polyethylene. The model couples torsional and longitudinal degrees of freedom and appears as an extension of a model that describes only the torsional motion. We find exact solutions that describe stable topological twistons, in good agreement with the torsional and longitudinal interactions in polyethylene.

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of -conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy (ACME) multiconfiguration self-consistent field (MCSCF) method and the Graphically Contracted Function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, the development of