I believe error is related to MODOS_CHECKPOINT in your CASTEP calculation is pointing to a missing or incorrectly specified checkpoint file. In MolPDOS calculations, MODOS_CHECKPOINT is a file used to store and retrieve molecular orbitals, which are necessary for projecting the Density-of-States (DOS) onto molecular orbitals.

Here's some suggestion you may try:

1. Checkpoint File Creation: The checkpoint file is typically generated during an initial calculation (such as an SCF calculation) and contains necessary information about the wavefunctions and orbitals. You need to run a standard CASTEP calculation first (usually a geometry optimization or an SCF calculation), and make sure that the checkpoint file is generated. In your input files for that calculation, ensure that WRITE_CHECKPOINT is set to true.

2. Specifying MODOS_CHECKPOINT: Once the checkpoint file is created, the MODOS_CHECKPOINT should refer to the correct file in your MolPDOS input. If the checkpoint file is named something like molecule.check, set the following line in your parameters file:

MODOS_CHECKPOINT = molecule.check

3. Lastly Ensure File Paths are Correct: Make sure the path to the checkpoint file is accurate in your input, especially if you're running the calculations in different directories.

Therefore, by generating the checkpoint file first and correctly pointing to it in the MolPDOS calculation, the error should be resolved.

Please let us know if it helps

Many thanks for your answer!

I wasn't indicating the correct file, because I had specified .param file:

deltascf_checkpoint :

And not the correct keyword:

modos_checkpoint :

The first is used to calculate deltascf calculations, while the latter is used for MolPDOS.

I thought deltascf_checkpoint was used for both deltaSCF and MolPDOS indistinctively. I don't think it's specified in the manual.

Thanks a lot for your answer.

I am glad that it solved your problem.