Hi, I managed to make CONVERSION_FACTOR definitions to work between to unit systems (atomic and SI). I definied atomic system by following the si example. I can manage to transform lengths (probably because they are base dimensions in both systems, but when I try to do the same for energies (that is a base_dimension only in the atomic system) I can't compile the line with the declaration "en3". The first question is why the first ...CONVERSION_FACTOR doesn't work in the two directions (as it did for lengths). second, why the second ...CONVERSION_FACTOR doesn't help to compile en3. BOOST_UNITS_DEFINE_CONVERSION_FACTOR( atomic::hartree_energy_base_unit, si::energy, double, 4.359744e-18 ); //compiles but doesn't help for en3 BOOST_UNITS_DEFINE_CONVERSION_FACTOR( si::energy, atomic::hartree_energy_base_unit, double, 1./4.359744e-18 ); int main(){ quantitysi::length l1(1.*si::meter); //works quantitysi::length l2(1.*atomic::bohr); //works quantityatomic::length l3(1.*atomic::bohr); //works quantityatomic::length l4(1.*si::meter); //works quantitysi::energy en1(1.*si::joules); //works quantitysi::energy en2(1.*atomic::hartree); //works quantityatomic::energy en3(1.*atomic::hartree); //work quantityatomic::energy en4(1.*si::joules); //does not work, compilation error return 0; } Thank you, Alfredo