pca(...)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()).   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.
pca.xyz, pca.pdbs,
  pdbaln.