A general fully automated implementation of the incremental scheme for molecules and embedded clusters in the framework of the coupled cluster singles and doubles theory is presented. The code can be applied to arbitrary order of the incremental expansion and is parallelized in a master/slave structure. The authors found that the error in the total correlation energy is lower than 1 kcal/mol with respect to the canonical CCSD calculation if the incremental series is truncated in a proper way.

Articles you may be interested inA sequential transformation approach to the internally contracted multireference coupled cluster method Block correlated coupled cluster theory with a complete active-space self-consistent-field reference function: The formulation and test applications for single bond breaking An exponential multireference wave-function Ansatz is formulated. In accordance with the state universal coupled-cluster Ansatz of Jeziorski and Monkhorst ͓Phys. Rev. A 24, 1668 ͑1981͔͒ the approach uses a reference specific cluster operator. In order to achieve state selectiveness the excitation-and reference-related amplitude indexing of the state universal Ansatz is replaced by an indexing which is based on excited determinants. There is no reference determinant playing a particular role. The approach is size consistent, coincides with traditional single-reference coupled cluster if applied to a single-reference, and converges to full configuration interaction with an increasing cluster operator excitation level. Initial applications on BeH 2 , CH 2 , Li 2 , and nH 2 are reported.

A systematic screening procedure for small contributions in the incremental expansion of the correlation energy is presented. The performance of the proposed scheme is checked for the calculation of intermolecular interactions in realistic test systems as large as a guanine-cytosine base pair. It is found that the computational cost for the incremental expansion can be reduced considerably without significant loss of accuracy. Typically, the errors of the systems investigated here amount to <3.4, 0.22, and 0.06% for second-, third-, and fourth-order expansions, respectively. For almost all cases, the error in the total correlation energy can be kept below 1 kcal/mol with respect to the canonical CCSD result if the incremental series is truncated in a proper way.

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The reaction mechanism and the energetics of the C2−C7 and of the C2−C6 cyclization of (Z)-1,2,4-heptatriene-6-yne are studied employing large scale MR-CI calculations. In addition the influence of vibrational motions and of temperature corrections on the barrier height of both reactions is estimated. For both products the first excited electronic states are computed to estimate possible switches from diradical to zwitterionic reaction mechanism.