Solvation of the Glycyl Radical.

The effect of adding explicit water molecules to the neutral (N) and zwitterionic (Z) forms of the glycyl radical has been examined. The results show that a minimum of three water molecules is required to stabilize the Z radical as a local minimum, with an energy gap of 123 kJ mol between the N and Z forms at this point, in favor of the N form. Increasing the number of water molecules to ∼20 leads to a converged Z-N energy difference of ∼50 kJ mol still in favor of the N form, even though the radical is not considered fully solvated from a structural point of view.

A systematic multireference perturbation-theory study of the low-lying states of SiC3.

The three known lowest-energy isomers of SiC(3), two cyclic singlets (2s and 3s) and a linear triplet (1t), have been reinvestigated using multireference second-order perturbation theory (MRPT2). The dependence of the relative energies of the isomers upon the quality of the basis sets and the sizes of the reference active spaces is explored. When using a complete-active-space self-consistent-field reference wave function with 12 electrons in 11 orbitals [CASSCF (12, 11)] together with basis sets that increase in size up to the correlation-consistent polarized core-valence quadruple zeta basis set (cc-pCVQZ), the MRPT2 method consistently predicts the linear triplet to be the most stable isomer.

Multireference second-order perturbation theory: how size consistent is "almost size consistent".

A systematic study of the deviation from size consistency of the multireference second-order Moller-Plesset perturbation theory (MRMP2) method is presented. The size-consistency error is shown to depend on the number of monomers in a supermolecule calculation, size of basis set, number of correlated valence electrons, and size of active space. HF, F(2), and N(2) are used as test cases, with stretched bonds, to include simple, well-defined multireference character.