Pathology m Force-Field Computation Associated with Hartree Fock Symmetry-Breaking Instability

Energy gradients are easy to compute, in the restricted SCF model, provide an insight into reaction mechanisms, and make possible the efficient computation of force constants, equilibrium geometries, and the structures of transition states.3- 6 However, there are some serious hazards in the routine use of the gradient programs now becoming widespread, associated with the instability of symmetry- restricted SCF solutions to symmetry-breaking. Lowering of the electronic energy associated with the symmetry breaking· in the wave functions may be induced deliberately by choice of the basis set, or unintentionally by the nuclear distortions which are introduced in the course of calculations of force constants. If the latter, · an artificially low, and perhaps negative force constant will be computed, associated with a »normal coordinate« which breaks the molecular symmetry. This will produce false conclusions on the ease of symmetry breaking distortions. We show by reference to a model two-electron system that this problem is frequently encountered in twixtyl or diradical systems, and can be overcome by choosing a wavefunction which is not symmetry or spinrestricted, without the need for extensive configuration mixing

Managing Deer Damage in Nebraska

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Phenylene coupling of methylene sites. The spin states of bis(X-methylene)-p-phenylenes and bis(chloromethylene)-m-phenylene

We judge the energetic sequence of spin states in substituted methylenes by ab initio multiconfigurational computations and, where feasible, density functional modeling techniques. The best of these calculations reproduce well-established singlet-triplet gaps in X-C-Y species, in which X can be phenyl and Y can be H, methyl, or chloro. Similar computations on p-phenylene-coupled Y-methylenes and meta-coupled Y-methylenes support the suggestion by Zuev and Sheridan that bis(chloromethylene)-p-phenylene has a singlet diradical ground state. However, despite the density functional computations' support for those authors' suggestion that bis (chloromethylene)-m-phenylene has a singlet ground state, we find that our best MCSCF calculations place the quintet ground state suggested by the simplest theory almost equal in energy to that singlet