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The main objective of this study consists in providing reliable structures, harmonic vibrational frequencies, and relative electronic energies of Si, GeSi, GeSi, and Ge clusters using DFT, CCSD(T), CASSCF, and MP2 methods and extended basis sets. The CCSD(T) results are extrapolation to the complete basis set (CBS) limit. For Si cluster, the ground-state is a singlet (A) and the lowest lying electronic state is triplet state ( ) separated by 0.44 kcal mol at the CCSD(T)/(CBS) limit. The ground-states of GeSi, GeSi, and Ge clusters are also singlet state with triplet states very close energetically. Computed equilibrium geometries and vibrational frequencies are compared with previous theoretical and experimental data. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Accurate potential energy curves, dipole moment functions, dissociation energies, and molecular constants for several low-lying singlet and triplet electronic states of BeS were investigated using the CASSCF/MRCI methodology, and the cc-pV5Z basis set for beryllium, and the aug-cc-pV(5+d)Z set for sulfur. Besides presenting improved results for the three lowest lying states, this study presents the first theoretical characterization of another set of nine excited states so far unknown experimentally. Our results are sufficiently accurate to reliably guide the experimental search and characterization of these states, and also to confirm the experimental assignment of the BΣ excited state. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Abstract: Geometries, energies, frequencies, and rate constants of the reaction F (2P)+C2H6 →C2H5 +HF were computed using the MP2 and CCSD(T) approaches, correlation-consistent basis sets, and an extrapolation scheme to assess the complete basis set (CBS) limit energies. Values of the enthalpy of reaction at 0K calculated with the CCSD(T)/cc-pVQZ method and extrapolated to the CBS limit are equal to −35.2 and −36.7kcal/mol, respectively (Exp. −35.3±0.5kcal/mol). Rate constants calculated with the zero-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. [Copyright &y& Elsevier]

Abstract: Equilibrium geometries, harmonic vibrational frequencies and the barrier heights of the internal rotation and inversion motion of methylamine (CH3NH2) conformers were calculated with the CCSD(T) method, in combination with the correlation consistent basis sets of Dunning. Valence and all electron correlation calculations were also carried out using the correlation consistent core and valence polarization triple-ζ basis set (cc-pCVTZ). Theoretical equilibrium geometries are in a general good agreement with available microwave and electron diffraction measurements. The best estimate results of the barrier heights for the internal rotation and inversion are, respectively, equal to 1.936 and 5.406kcal/mol. [Copyright &y& Elsevier]

Abstract: The CH4 +F(2P)→CH3 +HF abstraction reaction was investigated using the BH&HLYP, MP2, and CCSD(T) approaches, and two series of correlation consistent basis sets. At the CCSD(T)/cc-pVTZ level, the classical barrier height (ΔV #), and the vibrationally adiabatic ground-state barrier height () are equal to 1.8 and 0.5kcal/mol, respectively. CCSD(T)/cc-pVQZ calculations give a value for the enthalpy of reaction at 0K () of −31.5kcal/mol (Expt. −32.1kcal/mol). Rate constants calculated with the zero-order interpolated variational transition state (IVTST-0) method are in agreement with experiment. The activation energy computed as local two-points slopes of Arrhenius plots in the temperature range of 200–400K, equal to 0.59kcal/mol, is also in good agreement with experiment (0.53kcal/mol). [Copyright &y& Elsevier]

Abstract: Accurate potential energy curves, dissociation energies and molecular constants for several low-lying singlet and triplet electronic states of MgS were investigated using the CASSCF/MRCI methodology, and the cc-pVQZ basis set. Our results represent an improvement over previous theoretical description for the lowest lying states, characterize some higher excited states not previously investigated, and also confirm the assignment of the B1Σ+ excited state investigated experimentally. Transition probabilities and radiative lifetimes were also predicted using Einstein spontaneous emission coefficients. [Copyright &y& Elsevier]

Abstract: The lowest singlet and triplet states of AlP3, GaP3 and BP3 molecules with Cs, C2v and C3v symmetries were characterized using the B3LYP functional and the aug-cc-pVTZ and aug-cc-pVQZ correlated consistent basis sets. Geometrical parameters and vibrational frequencies were calculated and compared to existent experimental and theoretical data. Relative energies were obtained with single point CCSD(T) calculations using the aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets, and then extrapolating to the complete basis set (CBS) limit. [Copyright &y& Elsevier]