Calculation of isotropic Compton profiles with Gaussian basis sets

In this paper we present an adaptive algorithm for calculating the isotropic Compton profile (ICP) for any type of Gaussian basis set. The ICP is a measure of the momentum density of electrons and it can be obtained from inelastic X-ray scattering experiments employing synchrotron radiation. We have performed calculations of the ICP for water and helium monomers and dimers using density-functional theory, Hartree-Fock and post-Hartree-Fock methods, with Dunning-type ((d-)aug-)cc-p(C)VXZ basis sets. We have examined the convergence of the Compton profile as a function of the basis set and the level of theory used for the formation of the density matrix. We demonstrate that diffuse basis functions are of utmost importance to the calculation of Compton profiles. Basis sets of at least triple-zeta quality appended by diffuse functions should be used in Compton profile calculations in order to obtain sufficient convergence with regard to the current, experimentally feasible accuracy for systems consisting of light elements.