A contracted Gaussian basis set (6-311G**) is developed by optimizing exponents and coefficients at the Mo/ller–Plesset (MP) second-order level for the ground states of first-row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple molecules at various levels of MP theory and comparing with experiment.

AbstractsAn approximate fourth-order expression for the electron correlation energy in the MgllerPlesset perturbation scheme is proposed. It takes into account all the contributions to the fourthorder energy neglecting only those of the triple-substituted determinants. It is size consistent and correct to fourth order for an assembly of isolated two-electron systems. Illustrative calculations are reported for a series of small molecules.On propose une expression approchie de quatritme ordre pour l'tnergie de correlation Blectronique dans le schema de perturbation de Mgller-Plesset. Dans ce schema toutes les contributions a I'tnergie d'ordre quatre sont indues, sauf celles des determinants triplernent substituts. Le schema est consistent par rapport a I'ttendue des systtmes et correct jusqu'au quatrikme ordre pour un

The complete spin-orbital formulation of the analytical first and second derivatives of the Hartree-Fock (HF) energy as well as the analytical first derivative of the correlated second-order Mfller-Plesset perturbation energy (MP2) is presented. Some features of an efficient computational method to calculate these derivatives are described. The methods are applied to calculate the harmonic vibrational frequencies of ethylene, and the results are compared with experiment.

This paper serves two purposes. The first is to describe an implementation of the coupled cluster theory with double substitutions (CCD) previously developed by Cizek. The second is 10 apply this method and closely related fourth-order perturbation methods to some simple molecules and reaction potential surfaces. These studies show that CCD theory gives results close to those of a Mdler-Plesset perturbation treatment to fourth order in the space of double and quadruple substitutions MP4(DQ). Addition of contributions from single substitutions at fourth order makes little change in predicted relative energies. Preliminary results on the potential surfaces for 1 ,I-hydrogen shifts in CzH2, HCN, CH20, and N2H2 are discussed and compared with previous studies.

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