The LCD of 1/2 and 2/3 is 6.
You need at least two fractions to determine a least common denominator.
2
66
1122
198
286.
198
22
198
528
LCD(22, 11) = 22
There is none because the Least Common Denominator (or LCD) is the Least Common Multiple (LCM) of the denominators of two or more fractions.