Relative energies for different multiplets of trimethylenemethane and methylenecyclopropane are calculated at MCSCF, CASPT2N, and DFT levels of theory. Comparison to the experimentally measured heat of formation for the 3 A 2 ′ state and to the experimentally measured 3 A 2 ′-1 A 1 gap permits analysis of the relative importance of active space and basis set selection in the multiconfigurational methods. Such comparison also reveals that while DFT accurately treats triplet trimethylenemethane, there is a limitation in present DFT functionals with respect to accurately treating nondynamic correlation effects for closed-shell singlets in a molecule characterized by degenerate frontier molecular orbitals. Implications for calculations on larger systems are discussed.

No abstract

The interconversion of model compounds {[(NH3)3Cu]2(μ-η2:η2-O2)}2+ (1) and {[(NH3)3Cu]2(μ-O)2}2+ (2) has been examined using multireference second-order perturbation theory with an 8-electron/8-orbital active space. At this level of theory, 1 and 2 are separated by only 0.3 kcal/mol, and the barrier to isomerization is predicted to be very low based on single-point energy calculations for intermediate structures. The flat nature of the potential energy surface along the interconversion coordinate derives from a balancing of Coulomb forces and nondynamic electron correlation. The latter effect depends critically on the significant energy change experienced by the 13au virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1.

Stochastic search algorithms can be used to perform rapid sixdimensional molecular-replacement searches. A molecularreplacement procedure has been developed that uses an evolutionary algorithm to simultaneously optimize the orientation and position of a search model in a unit cell. Here, the performance of this algorithm and its dependence on search model quality and choice of target function are examined. Although the evolutionary search procedure is capable of ®nding solutions with search models that represent only a small fraction of the total scattering matter of the target molecule, the ef®ciency of the search procedure is highly dependent on the quality of the search model. Polyalanine models frequently provide better search ef®ciency than allatom models, even in cases where the side-chain positions are known with high accuracy. Although the success of the search procedure is not highly dependent on the statistic used as the target function, the correlation coef®cient between observed and calculated structure-factor amplitudes generally results in better search ef®ciency than does the R factor. An alternative stochastic search procedure, simulated annealing, provides similar overall performance to evolutionary search. Methods of extending the evolutionary search algorithm to include internal optimization, selection and construction of the search model are now beginning to be investigated.