The sign change o
f the intermolecular
2hJ(F,F) coupling in the (HF)
2 dimer as a
function o
f the F-F distanceis discussed by means o
f the CLOPPA method. It is
found that it is due to the competition o
f positive andnegative contributions involving the interaction o
f the
fchars/sigma.gi
f" BOR
DER=0 > lone pair o
f the acceptor nucleus with vacant molecularorbitals localized in the F-H···F moiety and with other molecular orbitals localized in the donor molecule.The origin o
f the sign o
f each contribution is
fully
determined by analyzing the response o
f the electronicsystem to the magnetic perturbation at the acceptor F nucleus.
2hJ(F,F) coupling in the FH···F
-, which ispositive
for all F-F distances, is also analyzed in or
der to look
for the di
fferences with the
former case.