The variance of the OLS intercept estimator is given by $Var(\hat{\beta}_0) = \sigma^2 \left[ \frac{1}{n} + \frac{\bar{x}^2}{\sum (x_i - \bar{x})^2} \right]$. Under what condition is the variance of the intercept estimator minimized, assuming the sample size $n$ and the spread of $x$ are fixed?