Similarity-transformed Hamiltonian calculations

The aim of our similarity-transformed Hamiltonian is to improve the computation of the correlation energy of conventional configuration-interaction (Cl) calculations. In this framework, the conventional wave function is multiplied by the correlation function 14,15,16)... 

CCSD/EOMCCSD equations defined by Eqs. (58) and (59). These moments are easy to calculate. As implied by Eqs. (58) and (59), their determination requires the explicit consideration of the triples-reference, triples-singles, and triples-doubles blocks of the matrix representing the CCSD/EOMCCSD similarity-transformed Hamiltonian Eq. (13). 

An interesting feature of the PE3 approximation, apart from its potential usefulness in the calculations (the T3-like terms analogous to T3 contributions to EOMCCSDT that enter the PE3 and PE3 approximations are the same see Section 6), is its clear relationship to the following decomposition of the similarity transformed Hamiltonian 77 >open,... 

A third procedure for calculating correlated frequency-dependent properties is the coupled-cluster, equations-of-motion (CC-EOM) method. This approach is formally equivalent to solving the sum-over-states expression using a similarity transformed Hamiltonian, H = e He, where the transformation is obtained from the coupled cluster choice for a reference state,... 

Figure A2.5 displays the results of carrying out the similarity transform of the final five orthonormal functions of hu irreducible symmetry on the Htickel Hamiltonian for the decorated regular orbit cage. Given the accuracy level applied in the calculations, with no precautions to ensure that notoriously unreliable functions are being calculated appropriately, the 3 figure accuracy in the overall diagonalization is acceptable.

The Fock space multireference CC methods and the intermediate Hamiltonian techniques (see e.g. Refs. [24-29] and references therein), as well as closely related similarity transformed EOMCC [30-33] are methods particularly suited for calculation of excited/ionized states with a multireference character. Recently, a Brillouin-Wigner formulation of Fock space CC has also been derived [34]. 

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