The intermolecular potential of HF

The functional form of separate contributions to the intermolecular potential between small dipolar molecules has been analysed using the HF intermolecular potential as an example. The SCF energy was partitioned into first order electrostatic and exchange terms and higher order terms, called the polarization energy, which can be obtained by iterating the wavefunction to self consistency. A comparison was made for each term between a bipolar expansion about a single centre in each molecule and spherically symmetric site-site models. The overlap dependent part of the electrostatic energy could be combined with the exchange energy to give a single term which we call the repulsion energy and this was most accurately represented by a six term bipolar expansion. The polarization energy has a long range (R ^-6) component and an overlap dependent component and both could be separately represented by a two-term bipolar expansion. A semiempirical estimate was made of the dispersion energy and the total potential predicts a geometry of the HF dimer in good agreement with experiment. The predicted binding energy of the dimer is 17 kJ mol^-1 which is slightly smaller than a previous empirical estimate (23 kJ mol^-1).