Formula Used
The calculator measures coordinate differences for every matched atom. It squares each three-dimensional displacement before averaging. The square root returns RMSD in the chosen distance unit.
Optimal alignment removes global translation and rotation before comparison. The quaternion form of the Kabsch method estimates rotation. Mass weighting replaces equal atom weights with atomic masses.
How to Use This Calculator
- Paste two coordinate sets or upload supported structure files.
- Select formats, PDB models, and alternate-location handling.
- Choose atom matching rules and molecular selection filters.
- Select alignment, weighting, output units, and precision.
- Optionally enter an outlier threshold for robust fitting.
- Press Calculate RMSD and review the result tables.
- Export summaries, detailed coordinates, or aligned PDB data.
Example Coordinate Data
| Atom | Residue | Reference coordinates | Comparison coordinates |
|---|---|---|---|
| N | ALA1 | 0.000, 0.000, 0.000 | 0.600, -0.300, 0.200 |
| CA | ALA1 | 1.458, 0.000, 0.000 | 1.946, 0.305, 0.100 |
| C | ALA1 | 2.050, 1.420, 0.000 | 1.941, 1.842, 0.150 |
| O | ALA1 | 1.350, 2.390, 0.000 | 0.912, 2.474, 0.050 |
Understanding All-Atom RMSD
All-atom RMSD summarizes structural separation using every included atom. Small values usually indicate closely related conformations. Larger values can reflect domain motion, flexible loops, or mismatched atoms.
Alignment strongly affects the reported value. Unaligned RMSD includes simple movement through coordinate space. Aligned RMSD emphasizes internal structural differences after removing rigid-body motion.
Standard and Mass-Weighted RMSD
Standard RMSD gives every atom equal influence. Mass-weighted RMSD emphasizes heavier elements during fitting and scoring. Hydrogen-rich regions therefore contribute less under mass weighting.
Atom Matching Matters
Correct correspondence is essential for meaningful RMSD calculations. Atom order works only when both files share identical ordering. Identity matching is safer for rearranged coordinate records.
Interpreting Results Carefully
RMSD is useful, but it compresses complex motion into one number. Local charts and residue summaries reveal where movement occurs. Biological interpretation should consider flexibility and experimental uncertainty.
Limitations
Symmetric atoms may require specialized permutation matching. Periodic trajectories may need prior coordinate unwrapping. Missing residues can also change which atoms enter comparison.
Frequently Asked Questions
What does RMSD measure?
RMSD measures the average three-dimensional separation between corresponding atoms.
Should structures be aligned first?
Yes, when internal conformation is the primary comparison target.
Can hydrogen atoms be excluded?
Yes. Hydrogen exclusion often improves compatibility between experimental structures.
What is a good RMSD value?
Acceptable values depend on molecular size, flexibility, and scientific purpose.
Why are atoms unmatched?
Names, residue identifiers, chains, or selected filters may differ.
What does mass weighting change?
It gives heavier atoms more influence than lighter atoms.
Can I compare multiple structures?
Yes. Batch mode creates a pairwise RMSD matrix.
Does outlier removal hide atoms?
No. Outliers are excluded only from fitting, not final scoring.
Can aligned coordinates be downloaded?
Yes. The aligned comparison coordinates export as a PDB file.