The second-order correlation energy of the ground state of the argon atoms is calculated using basis sets of Gaussian-type functions. It is demonstrated that accurate second-order correlation energies can be obtained by employing systematic sequences of even-tempered basis sets of Gaussian-type functions. The pair correlation energies are compared with those obtained by Jankowski et al. (1979) using basis sets of exponential-type functions.
The accuracy of electronic structure calculations made within the algebraic approximation (that is by making finite basis set expansions) is compared with the results of previously reported fully numerical calculations for diatomic molecules. Sequences of even tempered basis sets of atom-centred Gaussian-type functions are employed to explore the accuracy that can be achieved in matrix calculations for the H i , HeH+ and HeH2+ molecular ions and the H2 molecule. It is demonstrated that sub-pHartree accuracy can be achieved in energy calculations. The relevance of these results to the accuracy that can be achieved in studies of polyatomic studies is emphasised.
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