Many libraries mislabel them. The IR21531 has a lower dead time (0.65 µs vs 1.2 µs). For most half-bridge simulations, the standard IR2153 model works fine, but check your datasheet.

: C:\Program Files (x86)\Labcenter Electronics\Proteus 8 Professional\LIBRARY

: Community-maintained collections like Karan-nevage/PROTEUS-LIBRARY often host missing IC models. Search specifically for .LIB and .IDX files. How to Install Your Downloaded Library