Hamiltonian valence universal

By adopting the no-pair approximation, a natural and straightforward extension of the nonrelativistic open-shell CC theory emerges. The multireference valence-universal Fock-space coupled-cluster approach is employed [25], which defines and calculates an effective Hamiltonian in a low-dimensional model (or P) space, with eigenvalues approximating some desirable eigenvalues of the physical Hamiltonian. The effective Hamiltonian has the form [26]... 

A particular variant of the coupled cluster method, called Fock-space or valence-universal [49,50], gave remarkable agreement with experiment for many transition energies of heavy atoms [51]. This success makes the scheme a useful tool for reliable prediction of the structure and spectrum of superheavy elements, which are difficult to access experimentally. A brief description of the method is given below. A more flexible scheme with higher accuracy and extended applicability, the intermediate Hamiltonian Fock-space coupled cluster approach, is shown in the next section. 

Among the fundamental reasons is the dilemma that the most straightforward formulation of an extensive theory leads inevitably to the appearance of the intruder problem and that it is hard to eliminate this problem without violating extensivity. In fact extensivity requires a Fock space formulation with a rnultiplicatively separable wave operator [12, 87]. This means that one formulates the wave operator and an effective Hamiltonian for the full valence space, for all possible particle numbers, i.e. that one uses a so-called valence universal theory. However then one can generally not avoid that external orbitals (i.e. which are not not in the valence space) get energies close to those of valence... 

This technique has provided a potential for the carbon atom which is universal in the sense that it can be transferred with good accuracy in the whole series of hydrocarbon compounds. The main advantage of simulating the Fock operator by the kinetic operator plus the sum of atomic potentials is to produce approximate valence Fock Hamiltonians which can be solved with... 

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