Exploring Biorthonormal Transformations of Pair-Correlation Functions in Atomic Structure Variational Calculations

Abstract

Multiconfiguration expansions frequently target valence correlation and correlation between valence electrons and the outermost core electrons. Correlation within the core is often neglected. A large orbital basis is needed to saturate both the valence and core-valence correlation effects. This in turn leads to huge numbers of CSFs, many of which are unimportant. To avoid the problems inherent to the use of a single common orthonormal orbital basis for all correlation effects in the MCHF method, we propose to optimize independent MCHF pair-correlation functions (PCFs), bringing their own orthonormal one-electron basis. Each PCF is generated by allowing single- and double- excitations from a multireference (MR) function. This computational scheme has the advantage of using targeted and optimally localized orbital sets for each PCF. These pair-correlation functions are coupled together and with each component of the MR space through a low dimension generalized eigenvalue problem. Nonorthogonal orbital sets being involved, the interaction and overlap matrices are built using biorthonormal transformation of the coupled basis sets followed by a counter-transformation of the PCF expansions. Applied to the ground state of beryllium, the new method gives total energies that are lower than the ones from traditional CAS-MCHF calculations using large orbital active sets. It is fair to say that we now have the possibility to account for, in a balanced way, correlation deep down in the atomic core in variational calculations.

References in corpus (3)

• Development of a configuration-interaction + all-order method for atomic calculations
• Core-valence correlations for atoms with open shells
• Multiconfiguration electron density function for the ATSP2K-package

Cited by in corpus (8)

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• Reevaluation of the nuclear electric quadrupole moment for 87Sr by hyperfine structures and relativistic atomic theory
• MCDF-RCI predictions for structure and width of $K{\alpha }_{1,2}$ x-ray line of Al and Si
• Role of hyperfine interaction in Landé $g$-factors of ${}^3{P}_0^o$ clock states