A priori selected configuration interaction with truncation energy error, general sensitivity analysis and application to the Ne atom

The a priori selected configuration interaction (SCI) method [J. Chem. Phys. 125, 014107 (2006)] seeks to approximate full CI results by performing CI on quantitatively selected spaces while estimating truncation energy errors for the discarded spaces. New selection methods are here introduced increasing efficiency therefore improving accuracy for a fixed amount of computer time. SCI is explained within a historical context, from my first steps as a graduate student until present. An application to the ground state of Ne with an energy-optimized basis of 205 radial functions up to l=20 yields an energy upper bound E = −128.937477 au(Ne) thus recovering more than 99.97% of the estimated correlation energy. The sources of the remaining 0.03% of the correlation energy are mentioned and challenges to be met for the incorporation of the missing correlation energy into the wave function are outlined.