The DSSP program was designed by Wolfgang Kabsch and Chris Sander to standardize secondary structure assignment. DSSP is a database of secondary structure assignments (and much more) for all protein entries in the Protein Data Bank (PDB). DSSP is also the program that calculates DSSP entries from PDB entries.The DSSP program works by calculating the most likely secondary structure assignment given the 3D structure of a protein. It does this by reading the position of the atoms in a protein (the ATOM records in a PDB file) followed by calculation of the H-bond energy between all atoms. The best two H-bonds for each atom are then used to determine the most likely class of secondary structure for each residue in the protein.
This means you do need to have a full and valid 3D structure for a protein to be able to calculate the secondary structure. There's no magic in DSSP, so e.g. it cannot guess the secondary structure for a mutated protein for which you don't have the 3D structure.
Reff.- http://www.cmbi.ru.nl/dssp.html
BioDB Extractor (BDE) provides the utility to calculate the DSSP files for provided PDB files in batch mode.

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