Double occupancy
= Double occupancy
{title2=$D_i=\langle n_{i\uparrow}n_{i\downarrow}\rangle$}
Two <electrons> of opposite <spin> occupying the same <spatial orbital>. The mean occupation product measures its <probability> at a specified site. In the two-electron dimer, the sum over sites is also the <probability> of any doubly occupied site, since two sites cannot both be doubly occupied.