  We evaluate level spacing and smallest eigenvalue distributions of chiral
random matrix ensembles transiting from symplectic or orthogonal to unitary
symmetry classes with a crossover parameter rho. As expected from the effective
sigma model description, these results can be fitted perfectly to the
fundamental or adjoint staggered Dirac spectrum of SU(2) quenched lattice gauge
theory under the imaginary chemical potential (twisting) mu. The linear
dependence of the parameter rho on mu determines the pion decay constant F as
its proportionality constant.
