code: public static final double getPsi(AminoAcid a, AminoAcid b)
  			throws StructureException {
  		if ( ! isConnected(a,b)) {
  			throw new StructureException(
  					"can not calc Psi - AminoAcids are not connected!");
  		}
  
  		Atom a_N   = a.getN();
  		Atom a_CA  = a.getCA();
  		Atom a_C   = a.getC();
  		Atom b_N   = b.getN();
  
  		// C and N were checked in isConnected already
  		if (a_CA == null)
  			throw new StructureException(
  					"Can not calculate Psi, CA atom is missing");
  
  		return torsionAngle(a_N,a_CA,a_C,b_N);
  
  	}
docs: Calculate the psi angle.
  
  @param a
  an AminoAcid object
  @param b
  an AminoAcid object
  @return a double
  @throws StructureException
  if aminoacids not connected or if any of the 4 needed atoms
  missing