Evolutionary models taking into account radiative accelerations , thermal diffusion , and gravitational settling for 28 elements , including all those contributing to OPAL stellar opacities , have been calculated for solar metallicity stars of 0.5 to 1.4 \mbox { \it M } _ { \odot } .
The Sun has been used to calibrate the models .
Isochrones are fitted to the observed color-magnitude diagrams ( CMDs ) of M 67 and NGC 188 , and ages of 3.7 and 6.4 Gyr are respectively determined .
Convective core overshooting is not required to match the turnoff morphology of either cluster , including the luminosity of the gap in M 67 , because central convective cores are larger when diffusive processes are treated .
This is due mainly to the enhanced helium and metal abundances in the central regions of such models .
The observation of solar metallicity open clusters with ages in the range 4.8–5.7 Gyr would further test the calculations of atomic diffusion in central stellar regions : according to non-diffusive isochrones , clusters should not have gaps near their main-sequence turnoffs if they are older than \approx 4.8 Gyr , whereas diffusive isochrones predict that gaps should persist up to ages of \approx 5.7 Gyr .

Surface abundance isochrones are also calculated .
In the case of M 67 and NGC 188 , surface abundance variations are expected to be small .
Abundance differences between stars of very similar T _ { { \mbox { \scriptsize eff } } } are expected close to the turnoff , especially for elements between P and Ca .
Moreover , in comparison with the results obtained for giants , small generalized underabundances are expected in main–sequence stars .
The lithium to beryllium ratio is discussed briefly and compared to observations .
The inclusion of a turbulent transport parametrization that reduces surface abundance variations does not significantly modify computed isochrones .