Is there a file size limit for local CIF files?

Local files can be up to 5 MB. Compressed .cif.gz files are checked at their compressed size, so gzip lets you open considerably larger structures. Entries fetched by PDB ID are downloaded directly from RCSB.

Do I need to convert mmCIF to PDB format first?

No. MolViewer reads mmCIF directly, and it is the better format anyway: it carries more metadata and has no limit on atoms or chains.

Can I open a CIF file from a small-molecule crystal structure?

Not at the moment. Those files use fractional unit-cell coordinates, and MolViewer reads only the Cartesian coordinates used in PDBx/mmCIF. Export the molecule as SDF, MOL or XYZ from your crystallography software and open that instead.

Where does the file go when I open it?

Nowhere. The file is parsed in your browser and stays on your device, unless you choose to create a share link for a scene that includes a small local file.

Online mmCIF and CIF file viewer

MolViewer reads macromolecular CIF files (PDBx/mmCIF), the format the Protein Data Bank uses for its archive. Drag a .cif, .mmcif or gzip-compressed .cif.gz file onto the viewer, or type a PDB ID and MolViewer fetches the mmCIF from RCSB for you. It is free, runs in your browser and needs no install.

Open the SARS-CoV-2 spike (6VXX) from mmCIF

SARS-CoV-2 spike glycoprotein (6VXX) loaded from mmCIF, colored by chain

Why mmCIF

The legacy PDB format stores each atom on a fixed-width 80-column line. That design caps a file at 99,999 atoms and 62 chains, far too small for ribosomes, viral capsids and many cryo-EM structures. PDBx/mmCIF replaces fixed columns with named data items such as _atom_site.Cartn_x, so there is no practical limit on size. It became the standard archive format of the worldwide PDB (wwPDB) in 2014, and structures too large for the old format are distributed only as mmCIF.

Because MolViewer always downloads mmCIF from RCSB, large entries such as the SARS-CoV-2 spike 6VXX open by ID, with no conversion step.

What MolViewer reads from the file

Coordinates from the _atom_site loop, including element, residue, chain and B-factor.
Helices and sheets from _struct_conf and _struct_sheet_range, used by the cartoon and by the secondary-structure color scheme.
The first model of multi-model entries such as NMR ensembles.
One conformer: where atoms have alternate locations, the first (A) is kept.

Once loaded, you get the full toolset: cartoon, ball-and-stick, spacefill and surfaces; six color schemes; distance, angle and dihedral measurements; a clickable sequence viewer; PNG export and share links. AlphaFold models are also distributed as mmCIF, so the same viewer opens predicted structures too.

Macromolecular CIF versus small-molecule CIF

The CIF name covers two dialects. Crystallographic databases for small molecules and minerals publish core CIF files that give atoms as fractional coordinates within a unit cell. MolViewer does not read these: it needs the Cartesian coordinates of PDBx/mmCIF. For a small organic molecule, open an SDF or MOL file instead, or look it up by name, for example caffeine.

Frequently asked questions

Is there a file size limit for local CIF files?: Local files can be up to 5 MB. Compressed .cif.gz files are checked at their compressed size, so gzip lets you open considerably larger structures. Entries fetched by PDB ID are downloaded directly from RCSB.
Do I need to convert mmCIF to PDB format first?: No. MolViewer reads mmCIF directly, and it is the better format anyway: it carries more metadata and has no limit on atoms or chains.
Can I open a CIF file from a small-molecule crystal structure?: Not at the moment. Those files use fractional unit-cell coordinates, and MolViewer reads only the Cartesian coordinates used in PDBx/mmCIF. Export the molecule as SDF, MOL or XYZ from your crystallography software and open that instead.
Where does the file go when I open it?: Nowhere. The file is parsed in your browser and stays on your device, unless you choose to create a share link for a scene that includes a small local file.