Coupled cluster method potential energy surfaces

The method of moments of coupled-cluster equations (MMCC) is extended to potential energy surfaces involving multiple bond breaking by developing the quasi-variational (QV) and quadratic (Q) variants of the MMCC theory. The QVMMCC and QMMCC methods are related to the extended CC (ECC) theory, in which products involving cluster operators and their deexcitation counterparts mimic the effects of higher-order clusters. The test calculations for N2 show that the QMMCC and ECC methods can provide spectacular improvements in the description of multiple bond breaking by the standard CC approaches. 

Recent years have witnessed a considerable activity towards extending the standard single-reference coupled-cluster (CC) methods (1-9) to potential energy surfaces (PESs) involving bond breaking without invoking a multireference description (see, e.g., refs 9-31). Undoubtedly, it would be very useful if we could routinely calculate large portions of molecular PESs with the ease-... 

Figure 2. Comparison of the 2-RDM, coupled-cluster, MRPT2, and FCI potential energy surfaces of CO in a valence double-zeta basis set, where all valence electrons are correlated (a) without an electric field and (b) with an electric field of strength 0.10 an apphed in the direction of the permanent dipole moment. The 2-RDM and MRPT2 methods accurately describe the features of the FCI potential energy surface.

Stanton JF (1993) Many-body methods for excited state potential energy surfaces I. General theory of energy gradients for the equation-of-motion coupled-cluster method. J Chem Phys 99 8840-8847. 

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]. 

State Potential Energy Surfaces. I. General Theory of Energy Gradients for the Equation-of-Motion Coupled-Cluster Method. 

NUMERICAL RESULTS PROBING THE POTENTIAL ENERGY SURFACE OF THE WATER MOLECULE WITH THE STANDARD AND RENORMALIZED COUPLED-CLUSTER METHODS... 

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