Comments on the paper "Growth, structural, optical and thermal studies of semi-organic nonlinear optical potassium hydrogen oxalate single crystal".

The authors of the title paper (Appl Phys A 126:394, 2020) report to have grown a new nonlinear optical potassium hydrogen oxalate (KHO) single crystal, which exhibits second harmonic generation response. In this comment, it is shown that these claims are erroneous since KHO is a well-known solid which crystallizes in the centrosymmetric P21/c space group. [ABSTRACT FROM AUTHOR]