def HarmonicPairConstraints(a,b,value,sd):
    """
    Returns properly formated string for declaring harmonic distance constraints
    between CA carbons of resiedues a and b, at distance "value", and "sd" standard deviation.
    """
    st = "AtomPair CA %i CA %i HARMONIC %3.1f %3.1f\n" %(a,b,value,sd)
    return st