Performs principal components analysis (PCA) on biomolecular structure data.
pca(...)arguments passed to the methods pca.xyz,
    pca.pdbs, etc. Typically this includes either a numeric
    matrix of Cartesian coordinates with a row per structure/frame (function
    pca.xyz()), or an object  of class pdbs as obtained from
    function pdbaln or read.fasta.pdb (function
    pca.pdbs()).
Principal component analysis can be performed on any structure dataset of equal or unequal sequence composition to capture and characterize inter-conformer relationships.
This generic pca function calls the corresponding methods function for actual calculation, which is determined by the class of the input argument x. Use
  methods("pca") to list all the current methods for pca
  generic. These will include:
pca.xyz, which will be used when x is a numeric matrix
  containing Cartesian coordinates (e.g. trajectory data).
pca.pdbs, which will perform PCA on the 
  Cartesian coordinates of a input pdbs object (as obtained from 
  the ‘read.fasta.pdb’ or ‘pdbaln’ functions).
Currently, function pca.tor should be called explicitly as there
  are currently no defined ‘tor’ object classes.
See the documentation and examples for each individual function for more details and worked examples.
Grant, B.J. et al. (2006) Bioinformatics 22, 2695--2696.