The use of Gaussian orbitals in electronic structure theory was first proposed by Boys in 1950. The principal reason for the use of Gaussian basis functions in molecular quantum chemical calculations is the 'Gaussian Product Theorem', which guarantees that the product of two GTOs centered on two different atoms is a finite sum of Gaussians centered on a point along the axis connecting them. In this manner, four-center integrals can be reduced to finite sums of two-center integrals, and in a next step to finite sums of one-center integrals. The speedup by 4—5 orders of magnitude compared to Slater orbitals more than outweighs the extra cost entailed by the larger number of basis functions generally required in a Gaussian calculation. For reasons of convenience, many quantum chemistry programs work in a basis of Cartesian Gaussians even when spherical Gaussians are requested, as integral evaluation is much easier in the cartesian basis, and the spherical functions can be simply expressed using the cartesian functions.
Mathematical form
The Gaussian basis functions obey the usual radial-angular decomposition where is a spherical harmonic, and are the angular momentum and its component, and are spherical coordinates. While for Slater orbitals the radial part is being a normalization constant, for Gaussian primitives the radial part is where is the normalization constant corresponding to the Gaussian. The normalization condition which determines or is which in general does not impose orthogonality in. Because an individual primitive Gaussian function gives a rather poor description for the electronic wave function near the nucleus, Gaussian basis sets are almost always contracted: where is the contraction coefficient for the primitive with exponent. The coefficients are given with respect to normalized primitives, because coefficients for unnormalized primitives would differ by many orders of magnitude. The exponents are reported in atomic units. There is a large library of published Gaussian basis sets optimized for a variety of criteria available at the .
Molecular integrals
Taketa et al. presented the necessary mathematical equations for obtaining matrix elements in the Gaussian basis. Since then much work has been done to speed up the evaluation of these integrals which are the slowest part of many quantum chemical calculations. Živković and Maksić suggested using Hermite Gaussian functions, as this simplifies the equations. McMurchie and Davidson introduced recursion relations, which greatly reduces the amount of calculations. Pople and Hehre developed a local coordinate method. Obara and Saika introduced efficient recursion relations in 1985, which was followed by the development of other important recurrence relations. Gill and Pople introduced a 'PRISM' algorithm which allowed efficient use of 20 different calculation paths.
The POLYATOM System
The POLYATOM System was the first package for ab initio calculations using Gaussian orbitals that was applied to a wide variety of molecules. It was developed in Slater'sSolid State and Molecular Theory Group at MIT using the resources of the Cooperative Computing Laboratory. The mathematical infrastructure and operational software were developed by Imre Csizmadia, Malcolm Harrison, Jules Moskowitz and Brian Sutcliffe.
