Effective core potentials (ECPs) are widely-used in computational chemistry both to reduce the computational cost of calculations,(M. Dolg, 2000) and include relevant physics that would not otherwise be present (M. Dolg, 2002). In particular, for heavy main-group atoms (Wadt & Hay, 1985) and transition metals (Hay & Wadt, 1985), the number of core electrons greatly outnumbers the number of valence electrons. It is generally considered that these will not play a significant role in chemical reactivity, and thus can be frozen. Moreover, these electrons show significant relativistic character (M. Dolg, 2002; M. Dolg & Cao, 2012). Both of these issues can be resolved with the introduction of an effective core, represented as a fixed electronic potential. This potential is typically represented as a linear combination of gaussians of varying angular momenta (M. Dolg, 2000).
The introduction of an ECP results in an additional term in the core Hamiltonian, over which new electronic integrals must be computed. These three-center integrals are far from trivial, and they cannot in general be treated the same way as other electronic integrals (FloresMoreno et al., 2006; McMurchie & Davidson, 1981). Several widely used computational chemistry codes lack the ability to calculate these integrals due to the difficulty involved in their computation. The present library, libecpint, provides an open-source solution to this. It is a standalone library written in modern C++ capable of the highly efficient computation of integrals over ECPs with gaussian orbitals of arbitrary angular momentum, along with their first and second geometric derivatives. The methods implemented are based on novel algorithms that use automatic code generation and symbolic simplification of recursive expressions, along with highly optimised Gauss-Chebyshev quadrature.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)