Potential energy surface coupled-cluster CCSD

On the theoretical side the H20-He systems has a sufficiently small number of electrons to be tackled by the most sophisticated quantum-chemical techniques, and in the last two decades several calculations by various methods of electronic structure theory have been attempted [77-80]. More recently, new sophisticated calculations appeared in the literature they exploited combined symmetry - adapted perturbation theory SAPT and CCSD(T), purely ab initio SAPT [81,82], and valence bond methods [83]. A thorough comparison of the topology, the properties of the stationary points, and the anisotropy of potential energy surfaces obtained with coupled cluster, Moller-Plesset, and valence bond methods has been recently presented [83]. 

Maroulis and Haskopoulos calculated the interaction electric dipole moment and polarizability for the C02-Rg systems, Rg = He, Ne, Ar, Kr and Xe. The potential minimum is very well defined for all these systems. In Fig. 19 is shown the potential energy surface for the C02-He interaction calculated at the MP2 level of theory. The most stable configuration corresponds to a T-shaped structure. The two local minima for the linear configuration of C02-He are also clearly visible. All interaction induced properties were extracted from finite-filed Moller-Plesset perturbation theory and coupled-cluster calculations with purpose-oriented basis sets. CCSD(T) values were calculated for the dipole moment pim of C02-He and C02-Ne the corresponding results are 0.0063 and 0.0107 eao, respectively. All post-Hartree-Fock methods yield stable values for this important property. For C02-He, = 0.0070 (SCF), 0.0063 (MP2), 0.0063 (MP4),... 

Two alternatives to CASPT2 have recently appeared. One is the equation-of-motion (EOM) coupled-cluster method, which has been implemented in the ACES II program of Bartlett et al." This method, which can equally well be applied to open-shell molecules, has been reviewed, as already mentioned, in Volume 5 of this series.EOM-CC has been demonstrated to give results of similar accuracy as CASPT2 for states that do not have significant doubleexcitation character. A simplified version of this method, recently developed, can be applied to molecules up to the size of porphine. Also, it has been pointed out that because analytical gradients are available, the EOM-CCSD method is an excellent method for calculations of excited state potential energy surfaces. 

Calculations on selected benchmark atoms and molecules (Be, Ne, LiH, HF, H2O) have revealed the great potential of the coupled-cluster R12 method. The CCSD(T)-R12 calculations on the 10-electron systems represent the most accurate calculations at this level to date. At the second-order Moller-Plesset level, explicitly correlated wave functions have provided accurate and reliable potential energy surfaces for systems such as the HF dimer and the H2O trimer. 

Symmetry Specificity in the Unimolecular Decay of the CD CH3CI Complex Two-Mode Quantum Calculations on a Coupled-Cluster [CCSD(T)] Potential Energy Surface. 

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