Abstract The effects of core correlation on the bond distance and binding energy for the phosphorus dimer are investigated, and it is shown that these effects are substantial; the bond distance is reduced by almost 0.02 Å and the D e increased by 0.2 eV through core correlation. Two strategies are used to calculate the core-correlation contributions: explicit correlation of the (2s, 2p) core orbitals and use of the core polarization potential (CPP) method. In order to determine the CPP parameters, calculations of ionization potentials and some excitation energies for atomic phosphorus are also performed. Most of the calculations are done at the MR-ACPF level of correlation, using a CASSCF reference function, but also the CCSD, CCSD (T) and the recently developed CASPT2 methods are used. Different basis sets, ranging from an uncontracted (21s16p10d5f2g) basis to a [5s4p3d2f] ANO contracted basis, are used to investigate basis set effects. Our calculated values for the bond distance and the dissociation energy are 1.893 Å and 4.92 eV, respectively, which are in good agreement with the experimental results ( r e=1.893 Å, D e=5.08 eV).