Auxiliary basis expansions for large-scale electronic structure calculations

Abstract

One way to reduce the computational cost of electronic structure calculations is to use auxiliary basis expansions to approximate four-center integrals in terms of two-and three-center integrals, usually by using the variationally optimum Coulomb metric to determine the expansion coefficients. However, the long-range decay behavior of the auxiliary basis expansion coefficients has not been characterized. We find that this decay can be surprisingly slow. Numerical experiments on linear alkanes and a toy model both show that the decay can be as slow as 1͞r in the distance between the auxiliary function and the fitted charge distribution. The Coulomb metric fitting equations also involve divergent matrix elements for extended systems treated with periodic boundary conditions. An attenuated Coulomb metric that is short-range can eliminate these oddities without substantially degrading calculated relative energies. The sparsity of the fit coefficients is assessed on simple hydrocarbon molecules and shows quite early onset of linear growth in the number of significant coefficients with system size using the attenuated Coulomb metric. Hence it is possible to design linear scaling auxiliary basis methods without additional approximations to treat large systems.linear scaling ͉ resolution of the identity ͉ density fitting E lectronic structure calculations are normally performed by using basis set expansions to allow approximations to the Schrödinger equation to be expressed as algebraic rather than differential equations. Molecular electronic structure calculations (1) of either the density functional theory or wave-function type typically use standardized atom-centered basis sets, {͉ ͘}, whose functions are fixed linear combinations of Gaussian functions. With Gaussian basis functions, two-electron matrix elements,can be efficiently evaluated (2), normally with g(r 1 , r 2 ) ϭ ͉r 1 Ϫ r 2 ͉ Ϫ1 for Coulomb interactions. There are formally O(N 4 ) of these integrals for an atomic orbital basis set of size N. However, for a given choice of basis set, the number of nonnegligible integrals grows as only O(N 2 ) with increases in the size of the molecule. This growth arises from the rapid (Gaussian) decay of the amplitude of the product charge distribution ͉ ͘ ϵ (r 1 ) (r 1 ) with separation of the basis function centers. In density functional theory calculations, even this reduced bottleneck can be overcome for construction of the Coulomb matrix, J ϭ ͚ ͗ ͉ ͘P , from the density matrix by use of linearscaling fast multipole (3-5) and tree code methods (6).However, for a molecule of fixed size, increasing the number of basis functions per atom, n, does inexorably lead to O(n 4 ) growth in the number of significant integrals. This growth follows directly from the fact that the number of nonnegligible product charge distributions, ͉ ͘, grows as O(n 2 ). As a result, the use of large (high-quality) basis expansions is computationally costly. This article revisits perhaps the most practical way around this ''basis set quality'' bo...

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