Bond functions and core correlation energy contributions to HeBe potential

An empirical scheme for implementation of bond functions in heteronuclear diatomics is suggested and applied to HeBe using universal even-tempered functions. The effects of bond functions and core-correlation energy on the interaction potential of HeBe calculated at the uncorrelated (SCF) and correlated (MBPT and CC) levels are examined. The results confirm that an accuracy of sub mu Hartree level can be obtained using even-tempered functions with s-, p-, and d- symmetry and that bond functions of size \{4s2p\} for He and \{6s3p\} for Be recovers 100\% of energy lowering obtained from the addition of 10d atom-centered functions to He and 13d atom centred functions to Be. The various treatments of the electron correlation, conclude that the system is interacting weakly with a well depth from 14.5-24.7 mu E-h at a separation near 9.1a(o) compared with 20.7-25.5 mu E-h previously reported with a rather limited basis set. The most reliable well depth corrected for BSSE (19.0 mu E-h) was obtained at the CC-SD(T)level at separation of 8.71a(o) taking into account the effects of bond functions and core correlation energy. Potential energy curves at the CC-SD(T) valence and CC-SD(T) valence + core correlation levels are analyzed in analytical forms in terms of exchange repulsion, induction and dispersion components.