IO_U1_TO_FILE

The subkey IO_U1_TO_FILE determines whether the whole U1 matix that results from solving the coupled-perturbed Kohn-Sham (CPKS) is saved to file. If this key-word is not present then the whole U1 matrix is stored in memory, which can cause problems for large molecules. If this key-word is present, then only the required portion of the U1 matrix is held in memory, while the rest is stored on file.