The computational cost of performing a configuration interaction (CI) calculation for treating electron-electron correlation is directly proportional to the number of terms in the CI expansion. In this work, we present an <i>a priori</i> diagrammatic screening approach for the identification of non-contributing terms in a CI calculation. This method is based on using a R12-correlator operator for describing electron-electron correlation in an auxiliary many-electron wave function. The diagrammatic screening procedure is performed by first constructing all the Fermi vacuum polarization diagrams that arise from the overlap of the auxiliary wave function. In the next step, the vertex contributions of all the diagrams are evaluated using the R12-correlator operator and diagrams with contributions below a predefined tolerance value are eliminated. Finally, the action of the Hamiltonian operator is obtained by performing a diagram insertion procedure on the set of screened Fermi vacuum polarization diagrams. The diagrammatic screening method was applied to an isoelectronic series of 10 electron systems (CH4, NH3, H2O, HF, Ne) and the results from the calculations show the reduction in the size of the CI expansion by a factor of 3-9 increases the ground state energies by 10<sup>-5</sup> Hartrees. These results demonstrate the ability of the diagrammatic procedure described above to identify non-contributing states. The diagrammatic screening procedure was also applied to investigate bond dissociation and the results from the calculations will be compared with FCIQMC results for C2 dissociation. This method can also be applied to coupled-cluster theory and its extension to CCD and CCSD will be presented. [Elward, J. M. Hoja, J. Chakraborty, A. "Variational solution of the congruently transformed Hamiltonian for many-electron systems using a full-configuration-interaction calculation" (2012)] [Bayne, M. G. Drogo, J. Chakraborty, A. "Infinite-order diagrammatic summation approach to the explicitly correlated congruent transformed Hamiltonian" (2014)]

Diagrammatic screening approach to configuration interaction calculations

251st National Meeting (2016)

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