Articles you may be interested inMøller-Plesset perturbation theory gradient in the generalized hybrid orbital quantum mechanical and molecular mechanical method This paper presents a way of improving second-order perturbation theory calculations by summing contributions of uncoupled excitations to infinite order. For problems involving molecular vibrations, the new theory is shown to give similar results to conventional second-order perturbation theory when the system treated has no near resonances but also to give accurate and stable results even very close to resonance. The new theory is tested by comparison to converged variational calculations for vibrational energy levels of formaldehyde, formaldehyde-d 2 , and two two-dimensional model subsystems based on formaldehyde.

A theoretical study of the electronic structure and spectroscopic properties of the low-lying electronic states of the molecule SiB A theoretical investigation of some lowlying electronic states of imidazoleThe recently formulated multiconfiguration-based unitary coupled electron pair approximation (UCEPA) is compared with muItireference configuration interaction (MR-CISD) calculations, including all single and double excitations, for the molecules in this study. The electronic states of the molecules in this study are not only of experimental interest, but represent a challenge to any formalism to accurately predict the energy separations of the lowlying electronic states. The equilibrium geometries and fundamental vibrational frequencies of the three lowest electronic states (i.e., IA I , 3A If, and IA ") of amino nit rene H 2 N 2 , and phosphinonitrene, H 2 PN, have been determined using a split-valence basis with polarization functions on the heavy atoms and a small complete active space self-consistent-field (CASSCF) description of the active space. Both MR-CISD and UCEPA calculations have been performed at the equilibrium structures using larger basis sets to accurately determine the relative energetics of the electronic states. The equilibrium geometries and vibrational frequencies of the two lowest electronic states (Le., I A ' and 3 A ") of phosphinocarbene, H 2 PCH, have been determined using a larger than double zeta basis set, augmented with polarization and diffuse functions, and a CASSCF description of the active space. Both MR-CISD and UCEPA calculations were performed on the equilibrium structures and predict that the singlet lies between 10.4 and 11.8 kcallmollower in energy than the triplet. The use of a generalized valence bond (GVB) reference function within UCEPA is introduced and is shown to be a useful approximation.

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