The libchkpt.a library is a collection of functions used to access the PSI3 checkpoint file (file32) - the file which contains all most frequently used information about the computation such as molecular geometry, basis set, HF determinant, etc. Previously, the checkpoint file was a fixed-format file which is accessed using the old PSI3 I/O system. However, this changed in the spring of 2002 to use the new libpsio.a I/O system to access the checkpoint file, and it is now free format. That is, any programmer can add content to the file at will. The old checkpoint file interface has been updated to access the new underlying I/O system. It is mandatory that the checkpoint file is accessed via the libchkpt.a functions only.