Comment on “ c -axis Josephson tunneling in d x 2 − y 2 -wave superconductors”

Without giving or citing a derivation, Maki and Haas [Phys. Rev. B 67, 020510 (2003)] presented a formula for coherent tunneling between identical ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductors twisted about the c axis, which they claimed accounted for Andreev reflections at the grain boundary. However, their formula does not reduce to known results in the appropriate limits, casting serious doubt upon its validity, and upon its applicability to the cross-whisker data on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ by Takano et al. [Phys. Rev. B 65, 140513 (2002)] they tried to fit.