Correlation effects in the neutral and ionized ground states of acetylene

Abstract

Correlation energies were calculated by “Effective Pair Correlation Energy” (EPCE) and minimal basis set configuration interaction (CI) methods for pairs of electrons in the occupied molecular orbitals for the (π 4 ) neutral ground state, (π 3 ) ground state of the positive ion, and (π 4 π g ) ground state of the hypothetical negative ion of acetylene. The EPCE values allow detailed breakdown of the ionization potential and electron affinity (for the unstable negative ion). It is seen that the SCF values for the former can be modified by the EPCE values to give estimates close to the experimental quantity. The EPCE values are compared against the pair-wise correlation energies obtained by minimal basis set CI and the percentages of the latter as compared to the former are interpreted by considering the form of the available excited configurations used in constructing the correlated functions. The MBSCI calculation accounts for only 20–30 % of the EPCE correlation energy.